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plasma
Article
Physical Properties of Plasma-Activated Water
Mobish Shaji 1 , Alexander Rabinovich 1, *, Mikaela Surace 1 , Christopher Sales 2 and Alexander Fridman 1
1 C and J Nyheim Plasma Institute, Drexel University, Camden, NJ 08103, USA
2 Department of Civil, Architectural and Environmental Engineering, Drexel University,
Philadelphia, PA 19104, USA
* Correspondence: ar483@drexel.edu
Abstract: Recent observations of plasma-activated water (PAW)’s surfactant behavior suggest that
the activation of water with non-equilibrium plasma can decrease the surface tension of the water.
This suggested change to the surface tension also indicates that the addition of plasma can lead
to changes in the physical properties of the water, knowledge of which can expand existing PAW
applications and open new ones. While the chemical behavior of PAW has been extensively analyzed,
to the best of our knowledge the physical properties of PAW have not been investigated. This study
focuses on the need for experimental determination of PAW’s physical properties—namely, surface
tension, viscosity, and contact angle. The experimental results of this study show that the addition of
plasma lowers the surface tension of water at room temperature, increases the viscosity of water at
high temperatures, and lowers the contact angle of droplets on glass surfaces at room temperatures.
Potential factors influencing these changes include plasma alteration of the mesoscopic structure
of water at low temperatures and plasma additives acting as foreign particles in water at higher
temperatures. Ultimately, this investigation demonstrates that the physical properties of water change
due to plasma activation, which could lead to potential industrial applications of PAW as a surfactant
or as a washing-out and cleaning agent.
Keywords: plasma-activated water; PAW; physical properties; surface tension; viscosity; contact angle
Citation: Shaji, M.; Rabinovich, A.;
Surace, M.; Sales, C.; Fridman, A.
1. Introduction
Physical Properties of Non-thermal plasmas generated in ambient air, as well as in oxygen- or nitrogen-
Plasma-Activated Water. Plasma 2023, containing working gases and noble gases, produce a variety of reactive oxygen and
6, 45–57. https://doi.org/10.3390/ nitrogen species (RONS), e.g., ozone, hydroxyl radicals, hydrogen peroxide, superoxide,
plasma6010005 and nitrogen oxides. The interaction of plasma with liquid media leads to the transport
Academic Editor:
of RONS into the liquid and the formation of secondary active species. In water, the
Andrey Starikovskiy main reactive species formed by plasma activation are OH radicals, ozone, hydrogen
peroxide, nitrites, nitrates, peroxynitrites, and peroxynitrates; water activated in this way
Received: 22 December 2022 with non-thermal plasma is called plasma-activated water (PAW) [1]. PAW is considered to
Revised: 13 January 2023
be a green and prospective solution for numerous biotechnology applications, due to the
Accepted: 18 January 2023
transient nature of its biochemical activity. The biochemical activity of PAW is derived from
Published: 30 January 2023
the synergistic effects of active species—especially RONS. PAW currently has numerous
applications, including but not limited to surface disinfection, seed germination, use as a
fertilizer, inactivation of plant-based pathogenic organisms, curing fungus-infected plants,
Copyright: © 2023 by the authors.
food preservation, wound healing, deactivation of bacteria and viruses, mouthwash due to
Licensee MDPI, Basel, Switzerland. its bactericidal and fungicidal efficacy, selective killing of cancer cells, and insecticides [2–8].
This article is an open access article Plasma-activated water, because of its active biological properties, has been extensively
distributed under the terms and studied in order to understand plasma’s interactions with water [9], the production and behavior
conditions of the Creative Commons of active species [1], differences in the production of active species under varying plasma
Attribution (CC BY) license (https:// gases [8], identification and quantification of chemical species [3], transfer of specific active
creativecommons.org/licenses/by/ species—namely, RONS species—to the liquid [10], and differences in the production of active
4.0/). species with varied plasma systems and production parameters [11]. As a result of these detailed
Plasma 2023, 6, 45–57. https://doi.org/10.3390/plasma6010005 https://www.mdpi.com/journal/plasmaPage 2
Plasma 2023, 6 46
studies into the chemical behavior of PAW, there is an excellent understanding of PAW’s chemical
properties and biological behavior. For example, the antiviral behavior of PAW is attributed to
the short-lived ONOO- species [5], the plant growth improvement effect of PAW is attributed
to the aqueous nitrate, nitrite, and ammonium ions and hydrogen peroxide species [3], and
the antibacterial properties of PAW are attributed to the short-lived reactive oxygen species [4].
Until this study, the investigated physicochemical properties of PAW included pH, ORP, and
electrical conductivity; these properties were investigated in relation to the chemical properties
and behavior of PAW, as they are indicators of reactive species in the PAW [12].
While scientific interest and the number of publications related to plasma control of
the chemical properties of water are growing exponentially, to the best of our knowledge no
studies to date have focused on the physical properties and physical behavior of PAW. In a
recent study, J He et al. [13] observed that, while deactivating E. coli bacteria on the surface
of fresh produce, PAW significantly washed out or physically removed the E. coli from the
produce surface. While the RONS in PAW can deactivate bacteria, they cannot physically
remove them. This bacterial washing-out behavior of PAW is an indicator of its low
surface tension, or surfactancy. Therefore, these results suggest that the addition of plasma
brings about changes in the thermodynamic properties (e.g., surface tension) of water. If
the surface tension of the water is reduced via, plasma activation as suggested by J He
et al. [13], the potential of PAW as a surfactant widens quickly, as there are numerous fields
that could benefit from a liquid with anti-pathogenic and surfactant properties. Potential
applications for PAW as a surfactant might include use as a washing-out and cleaning agent,
an ingredient for eco-friendly detergents and soaps, surfactants for biomedical applications,
and more. Furthermore, since we have an indication that the addition of plasma can change
the surface tension of water, other physical properties of water could also be affected by the
addition of plasma. Investigating these properties is important to finding areas of potential
PAW applications.
Surface tension is the measure of force acting at the boundary of two phases. It refers
to the elastic tendency of fluid surfaces that makes them acquire the least possible surface
area. At the liquid–air interface, the greater attraction of liquid molecules to one another
than to air molecules results in surface tension. Hence, the cohesive force between the
liquid molecules is higher than the adhesive force between liquid and air molecules. This
results in an inward force at the liquid surface that causes it to behave as if the surface
were covered with an elastic membrane. Because of the high attraction of water molecules
to one another, water has a high surface tension (0.0728 N/m at 20 ◦ C) compared to
many other liquids. The surface tension of a liquid can provide insights into its capillarity
behavior, surfactant properties, etc. As a liquid with growing uses in the cleaning and
agriculture industries, insights into the surface tension properties of PAW can be useful for
its economical industrial adoption.
Viscosity, or more precisely shear viscosity, is the property that defines the quantitative
relationship between the applied shear stress and the shear deformation rate in a fluid.
Qualitatively, viscosity indicates the resistance to flow of a fluid. Since viscosity is the
property that controls and quantifies the shear stress/shear rate behavior in fluids, it is
in many regards the most important physical property of a fluid. Viscosity is stated in
two different forms: the absolute or dynamic viscosity (µ), and the kinematic viscosity or
momentum diffusivity (ν), where ν = µ/ρ and ρ is the fluid’s density [14].
The contact angle is another fundamental property of interest when the interface between
two fluids is also in contact with a surface, e.g., a water drop resting on a leaf. The contact
angle is dependent on the surface energy of the solid and describes how liquids spread on a
surface—vital information for dynamic liquid–solid processes such as coating and painting. In
addition, precise measurements of the contact angle between a fluid–fluid interface and a solid
surface are critical to deduce the wetting and spreading characteristics of liquids on surfaces, as
well as to calculate the surface energy of a solid by measuring the contact angle of a series of
liquids on one type of surface. These surface properties are important when considering, for
example, the application of dyes to surfaces and pesticides to plants [15].Page 3
coating and painting. In addition, precise measurements of the contact angle between a
fluid–fluid interface and a solid surface are critical to deduce the wetting and spreading
Plasma 2023, 6
characteristics of liquids on surfaces, as well as to calculate the surface energy of a solid 47
by measuring the contact angle of a series of liquids on one type of surface. These surface
properties are important when considering, for example, the application of dyes to sur-
faces and pesticides to plants [15].
Since studiesinvestigating
Since studies investigatingthe thephysical
physical properties
properties of PAW
of PAW (such
(such as surface
as surface tension,
tension,
viscosity, and
viscosity, contactangle
and contact angleare
arevery
verylimited),
limited), there
there is is a strong
a strong need
need to investigate
to investigate these
these
behaviors. Insights into the physical properties of PAW will provide users
behaviors. Insights into the physical properties of PAW will provide users with a better with a better
understanding
understanding of ofthe
thephysical
physicalbehavior
behavior ofof PAW
PAW andandof of plasma’s
plasma’s withwith
interaction
interaction waterwater
during
during PAW generation, and they will also help users identify properties that can be ap- be
PAW generation, and they will also help users identify properties that can
plied to to
applied present
present and and future
future areas
areas of need.
of need. In order
In order to address
to address thisthis
research gap,gap,
research this this
studystudy
focuses the changes
focuses on the changesoccurring
occurringininthethephysical
physical properties
properties of water
of water as aasresult
a result of plasma
of plasma
activation—namely,
activation—namely, the thesurface
surfacetension,
tension,viscosity,
viscosity,and
andcontact
contact angle
angle of of PAW.
PAW.
2.
2. Materials andMethods
Materials and Methods
2.1.
2.1. PAW
PAW Production
Production
The plasma-activated
The plasma-activated water water used
usedininthis
thistudystudywaswasproduced
produced with gliding
with arc plasma.
gliding arc
Gliding
plasma. arc plasma
Gliding arc is transient
plasma non-equilibrium
is a transient type oftype
non-equilibrium discharge with awith
of discharge relatively
a rela-high
microarc temperature (about 1600–1800 ◦
tively high microarc temperature (about K). In the gliding
1600–1800 °K). Inarc theplasmatron
gliding arc(Figures 1 and 2),
plasmatron
air is injected
(Figures 1 andtangentially
2), air is injectedinto the gap between
tangentially twogap
into the cylindrical
between electrodes, and elec-
two cylindrical a vortex
of air isand
trodes, created in the
vortex gap.
of air As energy
is created in theisgap.
supplied
As energybetween the high-
is supplied and the
between low-voltage
high-
electrodes,
and low-voltagethe plasma discharge
electrodes, occurs
the plasma betweenoccurs
discharge the electrodes.
between the The air vortex
electrodes. stretches
The air
and
vortexrotates the gliding
stretches and rotatesarc, the
thereby producing
gliding arc, thereby theproducing
plasma zone inside the
the plasma plasmatron.
zone inside the As
plasmatron.
water As water
is injected is injected
into the plasmatron into axially
the plasmatron
from theaxially
top, the fromwaterthedroplets
top, the react
waterwith
droplets react with the air that is coming out of the plasmatron,
the air that is coming out of the plasmatron, producing PAW. After being processed producing PAW. After
in the
being processed
gliding in the gliding
arc plasmatron, differentarc kinds
plasmatron,
of activedifferent
species kinds
(such OH species
active
of as radicals,(such as
hydrogen
OH radicals,
peroxide, NOx,hydrogen
etc.) areperoxide,
produced NOx, etc.)
in the are produced
water. The pH of in the
PAW water. The as
is 2–2.9, of PAW is
pHopposed to the
2–2.9,of
5.5–6 asdistilled
opposedwater. to theThe
5.5–6 of distilledparameters
operational water. The of operational parameters were
the PAW production of theasPAW
follows:
production
60–100 mL/minwere as follows:
water flow60–100 mL/minSLPM
rate; 40–100 waterplasma
flow rate; air 40-100
flow rate. SLPMThe plasma air
experiment
flowperformed
was rate. The experiment
using 400–1900 was performed
W plasma using
power.400–1900
The W plasma
gliding arcpower. The gliding
plasmatron used for
arc plasmatron
PAW productionused for study
in this PAW production
is shown ininFigure this study
1, and is ashown
schematic in Figure
of the1,gliding
and a arc
schematic of the gliding arc plasmatron’s
plasmatron’s operational principle is shown in Figure 2.operational principle is shown in Figure 2.
Plasma 2023, 6, FOR PEER REVIEW 4
Figure 1. Gliding
Figure Glidingarc
arcplasmatron
plasmatronsetup.
setup.
Figure 2.
Figure 2.Working
Workingprinciple
principleof the gliding
of the arc plasmatron.
gliding arc plasmatron.
2.2. Surface Tension Measurement Method
For more than a century, a variety of techniques have been used to measure the in-
terfacial tension between immiscible fluid phases. When we discuss the interfacial ten-
sion between a liquid and a gas, we call it surface tension. The different surface tensionPage 4
Plasma 2023, 6 48
2.2. Surface Tension Measurement Method
For more than a century, a variety of techniques have been used to measure the
interfacial tension between immiscible fluid phases. When we discuss the interfacial
tension between a liquid and a gas, we call it surface tension. The different surface tension
measurements are described in detail by Drelich et al. [16]. The different types of surface
tension measurements can be divided into five groups: The first group of techniques directly
measure the surface tension with a microbalance. Examples from this group include the
Wilhelmy plate and Du Noüy ring methods. The second group of techniques determines
surface tension through direct measurements of capillary pressure. Examples from this
group include the maximum bubble pressure and growing drop methods. The third group
of techniques relies on the balance between surface tension forces and variable volumes
of liquid to determine the liquid’s surface tension. Examples from this group include
the capillary rise and drop volume methods. The fourth group of techniques is based on
fixing the volume of the liquid and measuring the distortion of a drop of the liquid under
the influence of gravity. Examples from this group include the pendant drop and sessile
drop methods. The fifth group of techniques, used to measure ultralow surface tensions,
involves distortion of the shape of the liquid using centrifugal force. The pendant drop
method was chosen for surface tension measurements in this study, as it is the simplest,
most robust, and most versatile method. Surface tension measurement using the pendant
drop method consists of suspension of a liquid droplet from a needle [17].
Pendant drop tensiometry using OpenDrop software was chosen as the surface tension
measurement method for this project because of its accuracy and open-source nature [16,17].
Pendant drop tensiometry is performed by generating droplets of the liquid to be analyzed
using a syringe pump, capturing an image of the generated droplet, and iteratively fitting
Plasma 2023, 6, FOR PEER REVIEW the Laplace equation on this droplet image using image-processing software to determine
Plasma 2023, 6, FOR PEER REVIEW the surface tension of the liquid. Images of Pendant drop tensiometry performed are shown
in Figures 3 and 4.
Figure3.3.3.
Figure
Figure Image
Image ofof
Imageaofaadroplet
droplet during
during
droplet the pendant
during drop drop
pendant
the the method.
method.
pendant drop method.
Figure4.4.OpenDrop
Figure OpenDrop software
software fitting
fitting a Laplace
a Laplace equation
equation over a over a droplet
droplet image. image.
Figure 4. OpenDrop software fitting a Laplace equation over a droplet image.
The accuracy of this method was validated by correction factors with respect to
image quality obtained from the image processing software, called Worthington and
The accuracy of this method was validated by correction factors with re
Bond numbers. The recommended numbers for accurate measurements are 0.7–0.8 fo
image quality
Worthington obtained
numbers from the
and 0.3–0.35 forimage processing
bond numbers [17]. software,
The averagecalled Worthing
Worthington andPage 5
Plasma 2023, 6 49
The accuracy of this method was validated by correction factors with respect to
image quality obtained from the image processing software, called Worthington and
Bond numbers. The recommended numbers for accurate measurements are 0.7–0.8 for
Worthington numbers and 0.3–0.35 for bond numbers [17]. The average Worthington and
bond numbers for all measurements in this study were 0.77 and 0.31, respectively. The error
in all instances was 0.01 for both Worthington and bond numbers. These measurements
were taken at a room temperature of 18 ◦ C.
2.3. Viscosity Measurement Method
Viscometers make use of the theoretical relationship between shear stress and strain
rate to measure viscosity. There are three types of viscometer: flow, drag, and resonant.
In flow-type viscometers, the rate of flow of the fluid in a tube or through an orifice is
measured, and the shear stress can either be calculated or estimated based on theory.
Examples of flow-type viscometers include capillary tube viscometers, with which shear
stress is calculated, and cup viscometers, with which shear stress is estimated. Flow-type
viscometers measure kinematic viscosity. Drag-type viscometers measure either the force
on an object as it moves at a specified rate in the fluid (for example, rotational viscometers)
or the time it takes for an object to move a specified distance through the fluid (for example,
falling objects and bubble tube viscometers). Drag-type viscometers measure absolute
viscosity. The third type of viscometer is the resonant or vibrational viscometer, which is
most commonly used for in-line process applications [14].
Among the commercially available viscometers, a capillary viscometer was chosen for
viscosity measurements in this study because it was cost-effective and readily available.
Capillary viscometers determine viscosity by measuring the liquid flow rate through a
Plasma 2023, 6, FOR PEER REVIEW capillary tube. These viscometers are typically made of glass and consist of a bulb reservoir
connected to the capillary tube. The operation of a capillary tube viscometer is based on
the Poiseuille model of laminar flow which, describes flow through a round pipe [14].
The capillary viscometer used for this study was a certified, calibrated, size 25 Cannon-
was measured
Fenske viscometer. from 10 to 40
The certified °C with
accuracy thedevice
of this helpisof0.16%.
a water
The bath. The
viscosity wasetups
measured from 10 to 40 ◦ C with the help of a water bath. The setups of the viscometer and
eter and water bath are shown in Figures 5 and 6, respectively.
water bath are shown in Figures 5 and 6, respectively.
Figure
Figure 5. Cannon- FenskeFenske
5. Cannon- viscometer.
viscometer.Page 6
Plasma 2023, 6 50
Figure 5. Cannon- Fenske viscometer.
Figure Viscosity
Figure6. 6. measurement
Viscosity setup. setup.
measurement
The Cannon-Fenske viscometer has two fluid reservoirs connected by a tilted capillary
tube. AThe Cannon-Fenske
measured volume of theviscometer has two
liquid to be analyzed fluid reservoirs
is added connected
to the lower reservoir. by a tilte
After
lary tube. A
equilibration at ameasured volume ofthethe
constant temperature, liquid
liquid to beupanalyzed
is drawn through theiscapillary
added tube
to the low
to fill the second reservoir until it overfills. The liquid is then
voir. After equilibration at a constant temperature, the liquid is drawn allowed to fall under theup thro
influence of gravity, and the time taken for the liquid meniscus to pass between two marks
capillary tube to fill the second reservoir until it overfills. The liquid is then all
in the viscometer (as seen in Figure 5) is noted as the efflux time. The kinematic viscosity
fall
of theunder the influence
liquid, which of gravity,
is the mathematical and ofthe
product thetime
effluxtaken
time andforcertified
the liquid meniscus
viscosity
betweenwas
constants, twocalculated
marks in forthe viscometer
each temperature(as seen in Figure 5) is noted as the efflux ti
point.
kinematic viscosity of the liquid, which is the mathematical product of the effl
2.4. Contact Angle Measurement Method
and certified viscosity constants, was calculated for each temperature point.
The most common contact angle measurement methods include the telescope–goniometer
method, Wilhelmy balance method, captive bubble method, tilting plate method, and the
2.4. Contact
more Angle Measurement
recently developed Method methods. Among these methods, the most
drop shape analysis
frequently
The used
mostis direct
common measurement
contactofangle
the contact angle by telescope–goniometer.
measurement methods include In the tel
this method, a direct measurement of the tangent angle is taken at the three-phase contact
goniometer method, Wilhelmy balance method, captive bubble method, tiltin
point on a sessile drop profile. Drop shape analysis of sessile drop also measures the tan-
method,
gent and
angle at the the more recently
three-phase developed
contact point drop
with the help shape images
of droplet analysisandmethods.
computer Amon
methods,
programs the
[18]. most frequently
A simplified used
experimental is of
setup direct measurement
the drop shape analysis of the contact
method suitable angle
scope–goniometer. In this method, a direct measurement of the tangent
for researchers was used in this study for the measuring contact angle [19]. Droplets com-angle is
prising 20 µL of PAW and distilled water on clean glass microscope slides were captured
as images, as shown below. The contact angle between the liquid droplets and the glass
was measured using OpenDrop image processing software [17]. The contact angle made
by 20 µL of distilled water on a glass slide was measured to validate the accuracy of the
measurement. The contact angle for distilled water determined by this method was 54.5◦ ,
which is consistent with the literature reports of ~55 ◦ [20]. The images of the PAW droplet
making a 31◦ contact angle on the glass surface and the distilled water droplet making a
54.5◦ contact angle on the glass surface are shown in Figures 7 and 8, respectively. The
measurements were conducted at a room temperature of 18 ◦ C.
The surface tension and contact angle investigations in this study were limited to room
temperature due to the limitations presented by the cost of systems required for accurately
measuring these properties at varying temperatures.Page 7
software [17]. The contact angle made by 20 µL of distilled water on a glass slide was
measured to validate the accuracy of the measurement. The contact angle for distilled
water determined by this method was 54.5°, which is consistent with the literature re-
ports of ~55 ° [20]. The images of the PAW droplet making a 31° contact angle on the glass
surface and the distilled water droplet making a 54.5° contact angle on the glass surface
Plasma 2023, 6 51
are shown in Figures 7 and 8, respectively. The measurements were conducted at a room
temperature of 18 °C.
Figure 7. PAW
PAW droplet
droplet making
makingaa31
31°◦ contact
contact angle
angle on
on aa glass
glass surface.
surface.
Figure 8.
Figure 8. Distilled water droplet ◦ contact angle on a glass surface.
making aa 54.5°
droplet making 54.5° surface.
3. Results
The surface tension and contact angle investigations in this study were limited to
3.1.
room Surface Tension of
temperature duePAW
to the limitations presented by the cost of systems required for
accurately measuring
The surface these
tension properties
of two at varying
PAW amples temperatures.
of pH 2.5 and pH 2.78 was measured using
pendant drop tensiometry, at a room temperature of 18 ◦ C. The results obtained are shown
3. Results
in Figure 9. The literature values of water surface tension [21] is also included as a reference.
The PAW atTension
3.1. Surface pH 2.5 of
had a surface tension of 68.7 mN/m, while the PAW at pH 2.78 had a
PAW:
surface tension of 68.6 mN/m—both lower than the surface tension of distilled water at
◦ CThe surface tension of two PAW samples of pH 2.5 and pH 2.78 was measured using
Plasma 2023, 6, FOR PEER REVIEW 18 reported in the literature, which was 73.1 mN/m. On average, the PAW displayed a
pendant drop tensiometry, at a room temperature of 18 °C. The results obtained are8
viscosity 6.1% lower than that of distilled water. The relative accuracy of the measurements
shown in Figure 9. The literature values of water surface tension [21] is also included as a
was determined to be 3.65%.
reference. The PAW at pH 2.5 had a surface tension of 68.7 mN/m, while the PAW at pH
2.78 had a surface tension of 68.6 mN/m—both lower than the surface tension of distilled
74
water at 18 °C reported in the literature, which was 73.1 mN/m. On average, the PAW
Surface Tension in mN/m
displayed
72 a viscosity 6.1% lower than that of distilled water. The relative accuracy of the
measurements was determined to be 3.65%.
70
68
66
64
PAW pH 2.5 PAW pH 2.78 DW Literature
Samples
Figure9.9.Surface
Figure Surfacetension
tensionof
ofplasma-activated
plasma-activatedwater
waterat
at18
18◦°C; thesurface
C; the surfacetension
tensionof
ofdistilled
distilledwater
water
(DW) at this temperature is shown for reference.
(DW) at this temperature is shown for reference.
Usingthe
Using themodified
modifiedfreefree energy
energy equation,
equation, J He
J He et et al.
al. [13] [13] suggested
suggested thatwashing
that PAW’s PAW’s
washing
out out (physical
(physical removal) removal)
of E. coli of E. coli is
is aided byaided by a reduction
a reduction in the surface
in the surface tensiontension
of waterof
water
with with
the the addition
addition of plasma.
of plasma. The surface
The surface tensiontension
of water of is
water is lowered
lowered by the by the transi-
transition of
tion of the crystalline mesoscopic structure of water to an amorphous mesoscopic struc-
ture with the addition of plasma. This transition in the water’s structure is aided by the
plasma lowering the mesoscopic transition temperature. The results of this study are
consistent with the suggestion of J He et al. [13], showing that the addition of plasmaPage 8
Plasma 2023, 6 52
the crystalline mesoscopic structure of water to an amorphous mesoscopic structure with
the addition of plasma. This transition in the water’s structure is aided by the plasma
lowering the mesoscopic transition temperature. The results of this study are consistent
with the suggestion of J He et al. [13], showing that the addition of plasma lowers the
surface tension of the water. As the addition of plasma changes the surface tension of water,
it also influences its thermodynamic properties.
The mesoscopic structure of normal water is crystalline at temperatures below 35 ◦ C,
and it transitions to being amorphous at temperatures between 35 and 60 ◦ C [13]. The
crystalline structure is characterized by high surface tension and high viscosity, while the
amorphous structure is characterized by lower surface tension and viscosity relative to the
crystalline structure. One example of differences in the physical properties of water with
different mesoscopic structures is that hot water is more effective in cleaning applications
than cold water, due to the lower surface tension or surfactancy of hot water’s amorphous
structure compared to cold water’s crystalline structure. As J He et al. [13] witnessed
surfactant behavior/low surface tension in water at low temperatures, they suggested
that the addition of plasma could possibly lower the temperature required for mesoscopic
structural changes in water. The theoretical proof presented [13] in support of their claim is
discussed later to enrich the understanding of PAW’s physical behavior.
Reduction in surface tension increases the surfactant behavior exhibited by a liquid,
making it more suitable for cleaning and removal of particles. Since plasma-activated
water is biodegradable, it can be suitable for cleaning applications without harming the
environment.
3.2. Viscosity of PAW
The mesoscopic structural changes and foreign plasma additives in PAW influence
its viscosity. The structure change at low temperatures should reduce the viscosity of
PAW, while the foreign plasma additive should increase its viscosity. The effect of foreign
plasma additives in increasing the viscosity of water is similar to how sand added to water
can affect its viscosity; foreign additives can lead to increased friction in the liquid flow,
resulting in higher viscosities. Since it is proposed that the addition of plasma will result in
mesoscopic structural changes in water at low temperatures, PAW should display lower
to almost identical viscosity relative to water at low temperatures. This is because the
viscosity-reducing effect of mesoscopic structural change at low temperatures can possibly
be countered by the viscosity-increasing effect of foreign plasma additives. If the proposed
mesoscopic structural change does not occur, PAW should have a higher viscosity at lower
temperatures because of the foreign plasma additives. The foreign plasma additives should
also cause PAW to have higher viscosity at higher temperatures. At temperatures above
35 ◦ C, normal water has an amorphous structure [13], and the only influence differentiating
the rheological behaviors of PAW and normal water is that of the plasma foreign additives.
The viscosity of PAW at pH 2.78 was measured from 10 to 40 ◦ C using the Cannon-
Fenske viscometer and water bath setup described earlier (as shown in Figure 6). The
viscosity results obtained for this temperature range are shown in Figure 10. The kinematic
viscosity of water from the literature is included for reference [22].
The kinematic viscosity of PAW at 10 ◦ C was 1.28 mm2 /s, compared to the 1.30 mm2 /s
kinematic viscosity of water at the same temperature. The viscosity of PAW was 1.3% lower
than that of distilled water at 10 ◦ C. The slightly lower viscosity of PAW at low temperatures
supports the mesoscopic structural changes in water proposed by J He et al. [13]. Under
normal conditions, PAW should have exhibited a higher viscosity, since the foreign plasma
additives have the natural effect of increasing viscosity. The amorphous structure of PAW
at low temperature might have countered the viscosity-increasing effect of the plasma
additives, resulting in a lower viscosity than that of distilled water.Page 9
At temperatures above 35 °C, normal water has an amorphous structure [13], and the
only influence differentiating the rheological behaviors of PAW and normal water is that
of the plasma foreign additives.
The viscosity of PAW at pH 2.78 was measured from 10 to 40 °C using the Can-
non-Fenske viscometer and water bath setup described earlier (as shown in Figure 6).
Plasma 2023, 6 53
The viscosity results obtained for this temperature range are shown in Figure 10. The
kinematic viscosity of water from the literature is included for reference [22].
1.4 DW Kinematic Viscosity
(Literature)
1.3
Kinematic Viscosity mm2/s
1.2 PAW pH 2.78 Kinematic Viscosity
1.1
1
0.9
0.8
0.7
0.6
10 20 40
Temperature in degree celesius
Figure 10.
Figure 10. Viscosity of PAW
Viscosity of from 10
PAW from 10 to
to40
40°C;
◦ C;the
theviscosity
viscosityofofdistilled water
distilled (DW)
water is shown
(DW) for
is shown
reference.
for reference.
The kinematic
With the increase viscosity of PAW at
in temperature from °C ◦was
10 10 1.28
C, the high /𝑠, compared
𝑚𝑚viscosity of normalto the 1.30
water,
𝑚𝑚 /𝑠 kinematic viscosity of water at the same temperature. The viscosity
caused by its crystalline structure, started to decrease due to the transition of its structure of PAW was
1.3%
to lower than
amorphous. Asthat of distilledstructure
the crystalline water at in 10water
°C. The slightly
began lower viscosity
to weaken, of PAW
the relatively lower at
low temperatures
viscosity of PAW, due supports the mesoscopic
to its amorphous structural
structure, becamechanges in water proposed
less pronounced by J He
and began to
et al. [13].
match Under normal
the viscosity conditions,
of normal water, asPAW should have
demonstrated about 15 ◦aChigher
at exhibited viscosity,
in Figure 10. since
the foreign plasma
With further additives
increase have the natural
in temperature ◦ C, he
from 15effect of higher
increasing viscosity.
viscosity The water
of normal amor-
phous
due to itstructure
crystalline ofstructure
PAW atbegan low to temperature
decrease as its might have transitioned
structure countered further
the viscosi-
into
ty-increasingresulting
amorphous, effect ofinthePAW plasma
havingadditives, resultingthan
higher viscosity in anormal
lower viscosity
water. The than that of
viscosity-
distilled water.
reducing effect of the amorphous structure in PAW no longer countered the viscosity-
increasing
With effect of its foreign
the increase plasma additives.
in temperature from 10 The °C, kinematic viscosityof
the high viscosity of normal 20 ◦ C
PAW at water,
was 1.04bymm 2 1.00 mm 2 ◦
caused its/s, while that
crystalline of normal
structure, waterto
started was
decrease due /s;toathe20 transition
C, the viscosity of PAW
of its structure
was 2.19% higher
to amorphous. Asthan that of normal
the crystalline water.in water began to weaken, the relatively lower
structure
Following
viscosity of PAW, thisdue
trend of increased
to its amorphous viscosity
structure,in PAW
became at higher temperatures,
less pronounced PAW at
and began to
40 ◦ C had a kinematic viscosity of 0.76 mm2 /s, while that of normal water was 0.66 mm2 /s,
match the viscosity of normal water, as demonstrated at about 15 °C in Figure 10.
meaningWiththat PAWincrease
further had a 12.8% higher viscosity
in temperature from than
15 °C,normal
the higher at 40 ◦ C. of
water viscosity 40 ◦ C,
At normal
the
waterhigh viscosity
due caused bystructure
to its crystalline the crystalline
began structure
to decrease wasas minimal
its structurein normal water,fur-
transitioned as
its mesoscopic
ther into amorphous,structure might have
resulting transitioned
in PAW very close
having higher to amorphous;
viscosity than normal water. the
therefore, The
higher viscosity demonstrated
viscosity-reducing effect of the foramorphous
PAW, whichstructure
also has anin amorphous
PAW no longer structure, could the
countered be
attributed to the foreign plasma additives.
viscosity-increasing effect of its foreign plasma additives. The kinematic viscosity of
PAW at 20 °C was 1.04 𝑚𝑚 /𝑠, while that of normal water was 1.00 𝑚𝑚 /𝑠; at 20 °C, the
3.3. Contact Angle of PAW
viscosity of PAW was 2.19% higher than that of normal water.
The difference
Following thisin contact
trend angles made
of increased by liquids
viscosity in PAWon aatsurface
higher helps us to understand
temperatures, PAW at
the changes in surface
40 °C had energy
a kinematic betweenofthe
viscosity liquids
0.76 𝑚𝑚 /𝑠,andwhile
the contacting surface.
that of normal water contact
Thewas 0.66
angles made by PAW at pH values of 2.47, 2.68, and 2.85 on a glass microscope
𝑚𝑚 /𝑠, meaning that PAW had a 12..8% higher viscosity than normal water at 40 °C. At slide were
inspected. The results obtained are summarized in Figure 11. The contact angle made
by a distilled water droplet of the same volume in the same setup is shown for accuracy
indication. The measurements were conducted at a room temperature of 18 ◦ C.
PAW droplets make smaller contact angles on glass surfaces than water droplets,
by an average of 20◦ , or 36%. At lower pH or higher plasma production power, PAW
makes smaller contact angles. Therefore, the addition of plasma increases the surface
energy during interaction between the glass surface and the water. The contact angle is
an indication of the adhesive and cohesive forces exhibited by the liquid. If the adhesive
force of a liquid is high relative to its cohesive force, the liquid will wet the surface more,
resulting in a lower contact angle. If the cohesive force of the liquid is high relative to
its adhesive force, the liquid will wet the surface less, resulting in a higher contact anglePage 10
40 °C, the high viscosity caused by the crystalline structure was minimal in normal water,
as its mesoscopic structure might have transitioned very close to amorphous; therefore,
the higher viscosity demonstrated for PAW, which also has an amorphous structure,
could be attributed to the foreign plasma additives.
Plasma 2023, 6 54
3.3. Contact Angle of PAW
The difference in contact angles made by liquids on a surface helps us to understand
the changes
formed on the insurface.
surface energy between
Since plasma the liquids
activation and the
resulted in contacting surface.
water forming The contact
smaller contact
angles made by PAW at pH values of 2.47, 2.68, and 2.85 on a glass microscope
angles, it increased the adhesive force of water during contact with glass. Therefore, slide were
plasma
activation might increase the adhesive forces of liquids on surfaces; this could be usefulby
inspected. The results obtained are summarized in Figure 11. The contact angle made in
a distilled
the surfacewater droplet
treatment of the for
industry same volume in requiring
applications the same setup
betterisadhesion
shown for byaccuracy in-
dyes, along
dication.
with otherThe measurements
applications were conducted
that require at a room
better wettability of temperature of 18 °C.
liquids to surfaces.
60
50
Contact angle in degrees
40
30
20
10
0
PAW pH 2.47 PAW pH 2.68 PAW pH 2.85 DW
Samples
Figure11.
Figure 11.Contact
Contact angle
angle made
made by
by PAW
PAWand
anddistilled
distilledwater
water(DW)
(DW)on
onglass
glassslides.
slides.
4. Discussion
PAW droplets make smaller contact angles on glass surfaces than water droplets, by
an average
The lowofsurface
20°, ortension
36%. Atexhibited
lower pHbyorwater higher plasma
after production
plasma activationpower, PAW makes
was attributed by J
smaller
He contact
et al. [13] angles.
to the Therefore,
plasma’s effect ofthe addition
lowering theofmesoscopic
plasma increases
transition the surface energy
temperature. The
during interaction
mesoscopic structure between
of water theatglass
lowersurface and theiswater.
temperatures The contact
crystalline, whichangle is an indi-
is characterized
cation
by of the
higher adhesive
surface andand
tension cohesive forces
viscosity. exhibitedas
However, adhesive past
liquid. If theincreases
thetemperature
bythe of
forcethe
a liquid is high
mesoscopic relative
transition to its cohesive
temperature, force, the structure
the crystalline liquid will ofwet
waterthechanges
surfacetomore, result-
amorphous,
which a lower
ing in is contact angle.
characterized by lowerIf the cohesive
surface tensionforceand
of the liquid JisHe
viscosity. high relative
et al. to its ad-
[13] suggested
hesive
that force,
plasma the liquid
activation will the
lowers wetemperature
the surfacerequired
less, resulting
for this in a highercausing
transition, contactPAWangle to
display
formedlow on thesurface tension
surface. Sinceand low viscosity
plasma activationatresulted
lower temperatures
in water forming compared
smallerto contact
normal
water.
angles,J itHeincreased
et al. [13]the
theoretically
adhesive forcedemonstrated
of water thatduringplasma
contactactivation lowers
with glass. the free
Therefore,
energy of water. This can be shown with the following equation, where
plasma activation might increase the adhesive forces of liquids on surfaces; this could be the free energy of
usefulF in
water is given by
the surface treatment industry for applications requiring better adhesion by
dyes, along with other applications F = U − that T σ+ + (1 −wettability
(qVsbetter
require q) Vr ) of liquids to surfaces.
= qEs + (1?− q) Er + (qVs + (1 − q)?Vr ) P (1)
4. Discussion q 1− q
+k B T q ln gs + (1 − q)ln gr
The low surface tension exhibited by water after plasma activation was attributed by
J He etF al.
where is the
[13]free
to energy of water,
the plasma’s U isofthe
effect internalthe
lowering energy of water,transition
mesoscopic σ is the entropy, q is the
temperature.
percentage of structured state, Es,r are the specific energies of the structured and random
The mesoscopic structure of water at lower temperatures is crystalline, which is charac-
states, respectively (Es < Er ), Vs,r are the specific volumes, gs,r are the statistical degeneracy
terized by higher surface tension and viscosity. However, as the temperature increases
values (gs << gr ), T is the temperature, P is the pressure, and k B is the Boltzmann constant.
past the mesoscopic transition temperature, the crystalline structure of water changes to
When ions are added to a fluid, it changes the fluid’s free energy. Free energy in the
amorphous, which is characterized by lower surface tension and viscosity. J He et al. [13]
presence of ions contains an additional term, the Debye–Huckel term (UDH ):
suggested that plasma activation lowers the temperature required for this transition,
M Ni z2i e2 κ 1
UDH = − ∑ (2)
i =1
2 4πe r e 0 1 + καi
M
2e2
κ2 = ∑ z2 n
e r e0 k B T i = 1 i i
(3)Page 11
Plasma 2023, 6 55
where i denotes the type of ion species, Ni is the ion’s concentration, αi is the ion’s radius,
zi is the charge of an ionic species, e is the charge of the electron, M is the total number of
ionic species, er is the dielectric permittivity of the medium, e0 is the dielectric permittivity
of a vacuum, and κ is the inverse of the Debye screening length. The ions are unlikely to
penetrate the clusters; thus, in the first approximation, their impact is proportional to the
percentage of water in the amorphous phase. From this, we can derive the free energy of
plasma-activated water (FDH ), modified with the Debye–Huckel term as follows:
FDH = F + (1 − q)UDH (4)
From Equation (4), we can see that the addition of plasma ‘favors’ the amorphous
state; thus, the transition temperature decreases.
PAW presents interesting behavior as a solution. An ideal solution is a solution whose
properties change in proportion to the concentration of solute added to it. As PAW displays
changes in properties (i.e., 6.1% for surface tension, 1.3–12.6% for viscosity, 36% for contact
angle) that are significantly higher than the concentration of active species added to it
(0.01%), we can conclude that PAW behaves non-ideally.
The low surface tension characteristic of PAW indicates potential surfactant behavior.
Surfactants are required for numerous industrial processes, including but not limited to de-
tergents, paints, food emulsions, biotechnological processes, biosciences, pharmaceuticals,
and cosmetic products. PAW can be an eco-friendly and cost-effective alternative to current
products used for these applications [23]. As PAW is antibacterial, antifungal, and has
demonstrated its ability to disinfect bacteria [4] from fresh produce, it can be an excellent
washing-out agent—an industrial process that prevents disease breakouts due to microbes
on produce. Surfactants with antibacterial and antifungal properties are used in biomedical
fields [23]; as PAW has these properties [4,7] and can act as a surfactant, it can potentially
be applied as a biomedical industrial surfactant. Surfactants are also important ingredients
for the preparation of detergents and cleaning agents. PAW with surfactant properties can
potentially be used in these processes as a biodegradable and eco-friendly ingredient [23].
One major advantage of using PAW is its biodegradability [1], which means that it can meet
the surfactant requirements for industrial processes without harming the environment.
The viscosity of PAW is influenced by the mesoscopic structural changes and the
presence of foreign plasma additives due to plasma activation. At low temperatures,
when the effect of the crystalline structure is dominant in water, PAW with its amorphous
structure has a slightly lower viscosity. As temperatures increase beyond low values, the
friction-inducing plasma additives result in higher viscosity in PAW when compared to
water. Until the crystalline structure of water is transitioned to an amorphous structure,
the viscosity of PAW is only nominally higher than the viscosity of water. However, as the
temperature increases beyond the mesoscopic transition temperature in water (35◦ C [13]),
and the water attains an amorphous structure, the friction-inducing plasma additives cause
PAW to have a significantly higher viscosity than water (12.8%). The high viscosity of PAW
can lead to higher shear force exerted by the liquid during its flow at high temperatures.
The higher shear force exerted by the flow of PAW on a particle in its path can lead to better
removal of particles when compared to normal water, making PAW a better cleaning agent
at higher temperatures. The higher viscosity of PAW can also cause it to form a thicker
boundary layer during flow, thereby reducing transfer losses and making it suitable for
augmented oil extraction.
In terms of contact angle, the addition of plasma caused water to form smaller contact
angles on a glass surface, indicating that the addition of plasma increases the surface energy
during interaction between water and a glass surface. The addition of plasma resulted in
increased wettability and increased adhesion of water to the glass surface. Increased wetta-
bility and adhesion are of great use for applications in the surface treatment industry. These
improved surface properties exhibited by PAW suggest that plasma activation of liquids
might result in improved adhesion and wettability by liquids thus activated, potentially
improving the adhesive behavior of paints, dyes, etc.Page 12
Plasma 2023, 6 56
5. Conclusions
The aim of this study was to experimentally investigate the understudied physical
properties of PAW and see the effects of the addition of plasma to application-oriented
physical properties—namely, surface tension, viscosity, and contact angle. This study
inspected the surface tension and contact angle of PAW at room temperature (i.e., 18 ◦ C)
and the viscosity of PAW between 10 and 40 ◦ C. The addition of plasma resulted in changes
to the physical properties of water, as observed and theoretically suggested by J He et al. [13].
The major conclusions drawn from this study are as follows:
1. The physical properties of water change with plasma activation.
2. As a solution, PAW behaves non-ideally; since the percentage changes occurring to
the physical properties with the addition of plasma are higher than the percentage of
plasma species added.
3. The addition of plasma lowers the surface tension of water by 6.1% at room temper-
ature; it also decreases the temperature required for the mesoscopic transition from
crystalline to amorphous structure, resulting in lower surface tension in PAW relative
to normal water.
4. The addition of plasma increases the viscosity of water by 12.8% at higher tempera-
tures; foreign plasma additives lead to this increased viscosity in PAW. The viscosity-
increasing effect of plasma additives is inhibited at low temperatures due to the PAW’s
amorphous structure.
5. The contact angle made by water on glass surfaces is reduced by 36% with plasma
activation; thus, the surface energy during the interaction of water with glass is
increased with plasma activation, thereby increasing the wettability and adhesion of
water to the glass surface.
6. The changes occurring to the physical properties of water with plasma activation can
be attributed to water attaining an amorphous structure at lower temperatures, as
well as the presence of plasma additives at higher temperatures.
Author Contributions: Conceptualization, A.R., C.S. and A.F.; formal analysis, M.S. (Mobish Shaji)
and M.S. (Mikaela Surace); investigation, M.S. (Mobish Shaji) and M.S. (Mikaela Surace); methodology,
C.S.; project administration, A.R. and A.F.; supervision, A.R.; writing—original draft, M.S. (Mobish
Shaji); writing—review and editing, A.R. and A.F. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Data Availability Statement: No new data were created or analyzed in this study. Data sharing is
not applicable to this article.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Zhou, R.; Zhou, R.; Wang, P.; Xian, Y.; Mai-Prochnow, A.; Lu, X.; Cullen, P.J.; Ostrikov, K.K.; Bazaka, K. Plasma-Activated Water:
Generation, Origin of Reactive Species and Biological Applications. J. Phys. Appl. Phys. 2020, 53, 303001. [CrossRef]
2. Gapper, C.; Dolan, L. Control of Plant Development by Reactive Oxygen Species. Plant Physiol. 2006, 141, 341–345. [CrossRef]
[PubMed]
3. Judée, F.; Simon, S.; Bailly, C.; Dufour, T. Plasma-Activation of Tap Water Using DBD for Agronomy Applications: Identification
and Quantification of Long Lifetime Chemical Species and Production/Consumption Mechanisms. Water Res. 2018, 133, 47–59.
[CrossRef] [PubMed]
4. Ma, R.; Wang, G.; Tian, Y.; Wang, K.; Zhang, J.; Fang, J. Non-Thermal Plasma-Activated Water Inactivation of Food-Borne
Pathogen on Fresh Produce. J. Hazard. Mater. 2015, 300, 643–651. [CrossRef] [PubMed]
5. Guo, L.; Yao, Z.; Yang, L.; Zhang, H.; Qi, Y.; Gou, L.; Xi, W.; Liu, D.; Zhang, L.; Cheng, Y.; et al. Plasma-Activated Water: An
Alternative Disinfectant for S Protein Inactivation to Prevent SARS-CoV-2 Infection. Chem. Eng. J. 2021, 421, 127742. [CrossRef]
[PubMed]
6. Li, Y.; Pan, J.; Ye, G.; Zhang, Q.; Wang, J.; Zhang, J.; Fang, J. In Vitro Studies of the Antimicrobial Effect of Non-Thermal
Plasma-Activated Water as a Novel Mouthwash. Eur. J. Oral Sci. 2017, 125, 463–470. [CrossRef] [PubMed]Page 13
Plasma 2023, 6 57
7. Ten Bosch, L.; Köhler, R.; Ortmann, R.; Wieneke, S.; Viöl, W. Insecticidal Effects of Plasma Treated Water. Int. J. Environ. Res. Public
Health 2017, 14, 1460. [CrossRef] [PubMed]
8. Rathore, V.; Nema, S.K. The Role of Different Plasma Forming Gases on Chemical Species Formed in Plasma Activated Water
(PAW) and Their Effect on Its Properties. Phys. Scr. 2022, 97, 065003. [CrossRef]
9. Bruggeman, P.; Leys, C. Non-Thermal Plasmas in and in Contact with Liquids. J. Phys. Appl. Phys. 2009, 42, 053001. [CrossRef]
10. Kawasaki, T.; Koga, K.; Shiratani, M. Experimental Identification of the Reactive Oxygen Species Transported into a Liquid by
Plasma Irradiation. Jpn. J. Appl. Phys. 2020, 59, 110502. [CrossRef]
11. Rathore, V.; Nema, S.K. A Comparative Study of Dielectric Barrier Discharge Plasma Device and Plasma Jet to Generate Plasma
Activated Water and Post-Discharge Trapping of Reactive Species: Physics of Plasmas: Volume 29, No. 3. Available online:
https://aip.scitation.org/doi/10.1063/5.0078823 (accessed on 10 January 2023).
12. Rathore, V.; Patel, D.; Butani, S.; Nema, S.K. Investigation of Physicochemical Properties of Plasma Activated Water and Its
Bactericidal Efficacy|SpringerLink. Available online: https://link.springer.com/article/10.1007/s11090-021-10161-y (accessed
on 10 January 2023).
13. He, J.; Rabinovich, A.; Vainchtein, D.; Fridman, A.; Sales, C.; Shneider, M.N. Effects of Plasma on Physical Properties of Water:
Nanocrystalline-to-Amorphous Phase Transition and Improving Produce Washing. Plasma 2022, 5, 462–469. [CrossRef]
14. Kutz, M. Handbook of Measurement in Science and Engineering; John Wiley & Sons: New York, NY, USA, 2016; Volume 3.
15. Huang, E.; Skoufis, A.; Denning, T.; Qi, J.; Dagastine, R.R.; Tabor, R.F.; Berry, J.D. OpenDrop: Open-Source Software for Pendant
Drop Tensiometry & Contact Angle Measurements. J. Open Source Softw. 2021, 6, 2604.
16. Drelich, J.; Fang, C.; White, C.L. Measurement of Interfacial Tension in Fluid-Fluid Systems. Encycl. Surf. Colloid Sci. 2002, 3,
3158–3163.
17. Berry, J.D.; Neeson, M.J.; Dagastine, R.R.; Chan, D.Y.; Tabor, R.F. Measurement of Surface and Interfacial Tension Using Pendant
Drop Tensiometry. J. Colloid Interface Sci. 2015, 454, 226–237. [CrossRef] [PubMed]
18. Yuan, Y.; Lee, T.R. Contact Angle and Wetting Properties. In Surface Science Techniques; Springer: Berlin/Heidelberg, Germany,
2013; pp. 3–34.
19. Lamour, G.; Hamraoui, A.; Buvailo, A.; Xing, Y.; Keuleyan, S.; Prakash, V.; Eftekhari-Bafrooei, A.; Borguet, E. Contact Angle
Measurements Using a Simplified Experimental Setup. J. Chem. Educ. 2010, 87, 1403–1407. [CrossRef]
20. Mohsin, H.; Sultan, U.; Joya, Y.F.; Ahmed, S.; Awan, M.S.; Arshad, S.N. Development and Characterization of Cobalt Based
Nanostructured Super Hydrophobic Coating. IOP Conf. Ser. Mater. Sci. Eng. 2016, 146, 012038. [CrossRef]
21. Vargaftik, N.B.; Volkov, B.N.; Voljak, L.D. International Tables of the Surface Tension of Water. J. Phys. Chem. Ref. Data 1983, 12,
817–820. [CrossRef]
22. Huber, M.L.; Perkins, R.A.; Laesecke, A.; Friend, D.G.; Sengers, J.V.; Assael, M.J.; Metaxa, I.N.; Vogel, E.; Mareš, R.; Miyagawa, K.
New International Formulation for the Viscosity of H2 O. J. Phys. Chem. Ref. Data 2009, 38, 101–125. [CrossRef]
23. Fatma, I.; Sharma, V.; Kumar, A. Application of Surfactants for Better Tomorrow. J. Phys. Conf. Ser. 2022, 2267, 012125. [CrossRef]
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