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Effect of Interaction of Nano-Particles and Surfactants on the Spreading Dynamics of Sessile Droplets A R Harikrishnan, Purbarun Dhar, Sateesh Gedupudi, and Sarit Kumar Das Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02123 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017
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Effect of Interaction of Nano-Particles and Surfactants on the Spreading Dynamics of Sessile Droplets
A R Harikrishnan1, Purbarun Dhar2, Sateesh Gedupudi1 and Sarit K Das 1, 2, *
1
Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai–600036, India
2
Department of Mechanical Engineering, Indian Institute of Technology Ropar, Rupnagar–140001, India
*Corresponding Authors: Electronic mail:
[email protected] Phone: +91-1881-242101
Graphical Abstract
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Abstract While a body of literature on the spreading dynamics of surfactants and a few studies on the spreading dynamics of nanocolloids exist, to the best of the authors’ knowledge, there are no reports on the effect of presence of surfactants on the spreading dynamics of nanocolloidal suspensions. For the first time the present study reports an extensive experimental and theoretical study on the effect of surfactant impregnated nanocolloidal complex fluids in modulating the spreading dynamics. A segregation analysis of the effect of surfactants alone, nanoparticle alone and the combined effect of nanoparticle and surfactants in altering the spreading dynamics have been studied in detail. The spreading dynamics of nanocolloidal solutions alone and of the surfactant impregnated nanocolloidal solutions are found to be grossly different and particle morphology is found to play a predominant role. For the first time the present study experimentally proves that the classical Tanner’s law is disobeyed by the complex fluids in the case of particle alone and combined particle and surfactant case. We also discuss the role of imbibitions across the particle wedge in the precursor film in tuning spreading dynamics. We propose an analytical model to predict the nature of dependency of contact radius on time for the complex colloids. A detailed theoretical examination of the governing factors, the interacting forces at the three phase contact line and the effects of interplay of surfactants and the nanoparticles at the precursor film in modulating the spreading dynamics has been presented for such complex colloids.
Keywords: Colloids, spreading dynamics, surfactants, Tanner’s law, nanofluids, sessile
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1. Introduction The process of dynamic wetting is a topic of interest in many industrial and engineering applications such as in the paint industry, coating process, printing and so on1-3. A sound knowledge of spreading dynamics is essential to achieve the desired quality and performance of such systems4. Nanofluids, due to its unique thermophysical properties, has gained popularity in the recent past and its potential applications in various engineering applications have been studied extensively during the past few decades5, 6. Due to its unique properties, there have been many fundamental studies directed at probing the fundamentals of such unique characteristics during the past few years7- 9. Considering the practical aspects and applicability into account in the above mentioned areas there is only a limited literature on understanding the wetting and spreading mechanisms in the case of such complex colloidal solutions10-13. The surfactants, due to their strong influence in altering the interfacial energy at multi phase interactions at interfaces, have been used as potential agents to modulate the spreading dynamics. A large amount of research has been directed at understanding the effects of surfactants in altering the wetting and spreading behaviour because of the interfacial adsorption characteristics14, 15. There are also studies which studies the effect of surfactants in altering the pinning and de-pinning behaviour of the colloidal droplets and hence by modulating the dried coffee ring deposits after evaporation16,17. The literature deals with hydrophilic to superhydrophobic surfaces of varying wettability18 and the effect of surfaces in modulating the spreading dynamics of aqueous surfactant solutions. Numerous theoretical and numerical studies have been conducted to understand the physics of the phenomenon19, 20
. Wasan and Nikolov were the first to point out that the nanofluids are capable of
enhancing the spreading due to the layering of nanoparticles at the three phase contact line 10. They showed the enhancement in spreading is due to the pressure arising out of the colloidal ordering in the confined region. Later Kodiparty et. al11 have theoretically studied the effect of disjoining pressure on the spreading behaviour. However a recent study by Lu et al.12 reports that the addition of nanoparticles inhibits the dynamic wetting as compared to the base fluid. The authors attributed the above findings to the increase in surface tension or viscosity. Moreover they concluded that if the effects of surface tension and viscosity are eliminated, the wetting characteristics are independent of the nature of nanofluid. Bou-Zeid et 3 ACS Paragon Plus Environment
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al.13 conducted an interesting study on biocolloid, i.e. on human blood. They studied the effect of relative humidity on the spreading and wetting dynamics. Lim et al.21 have observed that the particle removal efficiency of nanofluids is well correlated with the structural component of disjoining pressure at the wedge shaped film in front of the apparent three phase contact point. Vafaei et al. have conducted an experimental study on the triple line spreading of silver nanofluids on the quality of the printed electronic tracks22. Li et al. have performed a molecular dynamic simulation of the spreading behaviour of nanofluids. They performed a scaling analysis of different competitive mechanisms which describe the power relationship between the contact radius and the time23. The recent studies by the present authors have concluded the effect of surfactants in modulating the surface tension8 and wettability9 of such complex multi component systems. It has been observed that in the case of combined nanoparticle and surfactant, the enhanced interfacial transport of surfactants by nanoparticles decreases the surface tension further and enhances the wettability. Since the surface active agents are smart agents in modulating the interfacial properties, the presence of surfactants can alter the spreading mechanism to a great extent. To the best of the authors’ knowledge, there are no studies reported as yet on the effect of surfactant impregnated nanocolloidal suspensions, where nanocolloidal suspensions where surfactants are added to the dispersions which forms a spherical cap over the particle, on the spreading dynamics.
The present study focuses on the spreading dynamics of
nanocolloidal suspensions with and without the presence of surfactants. Also there are no experimental studies reported as yet on the deviations in the behaviour of these complex colloidal solutions from the classical Tanner’s law. The present study experimentally quantifies and theoretically explains the physics behind such deviations and the dependency of such deviation on the particle and surfactant concentration. The magnitudes of deviations from the Tanner’s law between the cases of only nanocolloids and surfactant impregnated nanocolloids are far apart. The spreading index in the case of nanocolloidal solutions of only particle showed a huge deviation from the Tanner’s law compared to the case of combined nanocolloidal solutions of particles and surfactants. A scaling analysis based on the relative magnitudes of the governing forces has been presented in the current study and it indicates the importance of structural component of disjoining pressure and gives insight on the extent of spreading at different particle concentrations. Also the present study theoretically examines the deeper physics behind the anomalous behaviour of spreading dynamics in the
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presence of nanocolloidal particles and surfactants and also on the governing forces that result in spreading dynamics.
2. Materials and methodologies The present experiments have been planned in such a way as to understand the contributing effects of different components affecting the spreading process and dynamics. Different components include surface topographical texture, wettability, the morphology of the nanoparticles, the nature of surfactant molecules involved, concentration of the particles and surfactants etc. Glass slides (Sigma Aldrich) which are rendered uniformly hydrophilic by exposing to argon plasma treatment are considered as the substrate in the present study. Deionized (DI) water (Millipore, 1–3 µS/cm), metal oxide nanoparticles Al2O3 (~45 nm, Sigma Aldrich, USA) with a spherical morphology and carbon based nanocolloidal systems, viz. multiwalled carbon nanotubes (MWCNT) (20–40 nm external diameter and aspect ratio ~100, Sisco Research Lab, India) with tubular morphology and in-situ prepared graphene nano sheets with flake like morphology have been considered in the present investigation. Graphene is prepared in-situ using the two step process; oxidation of graphite powder to graphite oxide (GO) invoking the modified Hummer’s method followed by reduction of GO into reduced GO. The detailed procedure is described in literature24. The prepared graphene is characterised using Raman spectroscopy as illustrated in Fig. 1 (c). The characteristic D and G bands are observed at ~ 1355 cm-1 and ~1605 cm-1 respectively which represents the defects and in-plane stretching in graphene. The ratio of intensity of 2D to G band is around 0.3 to 4.3 indicating 3-4 layers of graphene25. The presence of 2D band at ~2800 cm-1 confirms that the sample is few layers graphene. The present choice of metal oxide and carbon based nanoparticles ranges over various morphological structures. Figure 1 (a) shows the High Resolution Scanning Electron Microscopy (HRSEM) image of Al2O3 with spherical morphology. Figure 1(b) shows the HRSEM image of MWCNTs and the SEM image characterisation of the Graphene is illustrated in Fig. 1(c). In the present study sodium dodecyl sulphate (SDS) (anionic, 99% pure, Sisco Research Labs, India) and Cetyl trimethyl ammonium bromide (CTAB) (cationic, 99.5% pure, Sisco Research Labs, India) have been used as the surfactants.
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Figure 1: Scanning Electron Microscopy (SEM) characterisation of (a) Al2O3 nanoparticles (b) Multiwalled Carbon Nanotubes (MWCNT) and (c) Graphene nanoflakes. The inset illustrates the Raman Spectroscopic characterisation of Graphene.
The experiments are conceived so as to clearly demarcate and understand the contributing effect of surfactants alone, particles alone and combined particles and surfactants in modulating the spreading dynamics. The first set of experimental run is conducted with aqueous surfactant solutions at various surfactant concentrations ranging from as low as 0.25 times the Critical Micelle Concentration (CMC) to up to the micellar concentration. The CMC values are fixed from our previous reported literature8 and the surfactant concentration is expressed in a non-dimentionalised form throughout this paper in the form CS = C/CCMC for each of the surfactants considered in the present study. The next set of experiments is conducted with nanofluids of only particles without the aid of any surfactants to understand and study the effect of particles in altering the wetting dynamics. The final set of experiments is performed with nanocolloids impregnated with various surfactants to identify and bring out the combined effect of surfactant and particles. The metal oxide particle concentration is varied from 0.1 percent by weight up to 2.5 percent by weight and the concentration of the carbon based nanoparticles is varied from 0.01 percent by weight up to 0.1 percent by weight. Three normalised surfactant concentrations (CS) viz. 0.25, 0.5, 1.0 have been taken throughout the study for different particle concentrations at a given value of CS. The droplets are generated with an automated syringe pump and the volume of the droplet for all the fluids is about 5±0.3 microliters. The substrates were pre-cleaned by dipping in acetone solution and then DI water and then placing in oven with vacuum. All the experiments were carried out at 30±2 oC and under relative humidity conditions of 50±5%.
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The present study employed the sessile drop technique to measure the spreading dynamics using a standard goniometer (Holmarc Optomechatronics, India). The change in the apparent contact angle is observed with respect to time for the various cases of fluids employing a camera at 20 fps. The instantaneous apparent contact angles, base diameter and spreading velocity are determined by post processing the captured video frame image. Weighted amounts of particles are dispersed into the accurately measured volume of DI water in preparing the dispersions and are sonicated (Oscar Ultrasonics, India) for requisite amount of time. The zeta potential measurement of the colloidal samples showed excellent stability and it is observed that the magnitude of the zeta potential increases with increase in the surfactant concentration in the colloidal sample. Dynamic Light Scattering (DLS) study has been conducted on these colloids to understand the particle size distribution (Malvern Instruments). As the surfactant concentration increases, the DLS study reported an increment in the effective particle size which can be attributed to the fact that as the surfactant concentration increases, the Electrical Double Layer (EDL) thickness increases and which results in the increase in effective hydrodynamic diameter of the particle. The zeta potential results also indicate the thickening of the EDL with the increase in surfactant concentration.
3. Results and Discussion 3.1. Effect of base fluid, surfactants The first set of experiments involves water on uniformly hydrophilic glass substrate to understand the base case of spreading dynamics. Fig. 2(a) illustrates the nature of variation of dynamic contact angle and the base diameter of sessile droplet of DI water just after making contact with the glass substrate. It can be noticed that there is a sharp decrement in the dynamic contact angle during the initial stage of spreading followed by a steady exponential decay in the dynamic contact angle before reaching the equilibrium contact angle. The base diameter increases till the equilibrium contact angle and thereafter no noticeable change can be observed in base diameter. The change in the dynamic contact angle is expressed through a non-dimensional form of contact angle as: ∗ =
where represents the dynamic
contact angle, represents the equilibrium contact angle after which no further change in
contact base radius is observed, is the initial contact angle from the point of measurement.
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the diffusion and adsorption of the suspended phase will drive the spreading mechanism. The
extent of spreading is quantified and represented using a spreading factor as ∗ = . D
represents the base diameter of the sessile drop at any instant of time and represents the
contact diameter of the droplet just after coming in contact with the substrate as illustrated in Fig 2(b). The time the droplet takes to reach the maximum diameter (which also corresponds to the equilibrium contact angle ) is represented by equilibrium time .
Figure2. (a) The nature of variation of the contact angle and the base diameter (in mm) with time for water (b) Illustration of the droplet dynamics and nomenclature at different stages of spreading. The scale bar above first droplet image indicates 0.3 mm (c) Illustrates the variation of the normalised contact angle with time and the normalised spreading diameter with time for normalised surfactant concentration of Cs=0.25 and Cs=0.5 for SDS solution. 8 ACS Paragon Plus Environment
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The uncertainties are observed to be within ± 5% of the spreading diameter and the equilibrium spreading time
Figure 2(d) illustrates the nature of variation of the normalised contact angle factor ∗
and the spreading factor ∗ for two normalised SDS concentrations of Cs=0.25 and Cs=0.5 respectively. The normalization for surfactants is done with respect to the critical micelle concentration (CMC). The time scale to reach the equilibrium contact angle is much higher compared to that of water. As expected, the nature of variation of ∗ at a concentration of Cs
= 0.5 is much larger as compared to that at Cs=0.25. The increase in surfactant concentration has enhanced the spreading as it can be observed from the variation of spreading factor with time for two concentrations of surfactant. The diffusive surfactant molecule transport to the solid-liquid and liquid-gas interface results in the decrease of interfacial energy and enhances the spreading. Again the preferential adsorption to the interfaces depends on the nature and properties of the surfaces and also on the surfactant properties. There is no considerable difference in the value of as both normalised contact angle curves meet the time axis together. However, it is interesting to note that the normalised contact angle variation is a path function since the curves are different even though the time to reach equilibrium contact angle is almost similar. Hence the presence of foreign substances alter the nature of spreading dynamics through mechanisms such as Marangoni stresses arising out of the localised concentration difference of the foreign substances. In aqueous surfactant solution, a fraction of surfactant molecules adsorb at the interface according to the Gibbs adsorption isotherm resulting in the decrease of interfacial tension8, 9 which leads to the spreading mechanism. It is to be taken into consideration that the surfactant molecules will adsorb to both the liquidvapour interface (thereby resulting in the reduction of surface tension) and the solid-liquid interface (thereby resulting in the reduction of solid-liquid interfacial energy). This diffusion driven interfacial transport of surfactant molecules is the main mechanism driving the spreading in the case of surfactant solutions. The surface excess concentration (Г) at the interface depends on many factors such as temperature of the solution, nature and activity of surfactant molecule, concentration etc. as per the adsorption equation given by Gibbs 26:
Г =
( (
=
! ( ("
(1)
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where ‘Rgc’ is the universal gas constant, ‘T’ is the absolute temperature and ‘a’ is the activity. In the case of a univalent electrolyte and in dilute suspensions, the mean activity coefficient can be approximated to be equal to one so that the Gibbs adsorption equation can be expressed in terms of concentration in solution, ‘c’.
Typical value of is higher for surfactants as compared to that of water. This can be attributed to the slow diffusion driven transport of surfactant molecules to the interfaces which extends the spreading regime. Surfactant molecules are subjected to both solid-liquid and liquid-gas interface. These adsorption processes are slow and result in a dynamic interfacial tension. When surfactant molecules are added to it, it tries to adsorb at the interface and exhibit a dynamic surface tension due to the interfacial adsorption desorption process. This can happen at both solid-liquid interface and at liquid-gas interface hence by changing the interfacial energies of both the interfaces. This is a slow and transient process which takes time. Hence because of this slow transient dynamic surface tension, the wetting/ spreading changes and as the surface tension decreases with time, it results in more spreading over a period of time. Moreover the local concentration gradient will result in the Marangoni stresses which are absent in case of water. The typical spreading velocity encountered in all the sets of experiments are quite low and hence the bulk of the droplet may be safely assumed to be unaffected by the viscous shear forces. Hence the drop shape can be assumed to be a hemispherical cap, except for the microscale precursor film region extending beyond the contact line. The apparent macroscopic angle which is referred to in the present study can be defined as the angle between the extrapolation to the spherical cap solution and the base line where the droplet rests. The present study focuses on the change in the apparent macroscopic contact angle with time due to the changes in the microscopic thin film region.
3.2. Effect of nanoparticles alone Figure 3(a) illustrates the nature of variation of dynamic contact angle with time for 0.1 wt% Al2O3 and the corresponding changes in the base diameter for nanocolloidal solutions without
any surfactant. The nature of variation of non-dimensional contact angle ( ∗ with time for
different particle concentration is illustrated in Fig. 3(b). The nature of variation of ∗ is
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function of the particle concentration as can be observed from Fig. 3(c). The equilibrium time increases with the particle concentration and with increasing concentration, the increment follows the law of diminishing return. It can be observed that the time constant curve attains a plateau nature towards higher particle concentration. The nature of variation of the time constant can be directly attributed to the adsorption desorption characteristics at the interface as this acts as a driving force in enhancing the spreading. As reported by Wasan et al10 and from the observations from previous studies9, the nanoparticles are driven to the three phase contact point due to congregated adsorption towards both interfaces which converge at the contact line. The nanoparticle population near the triple point line forms layering of nanoparticles.
It can be observed directly from Fig. 3(b) that the nature of variation of ∗ is entirely different from that of the only surfactant case as illustrated in Fig. 2(c). The normalised contact angle follows an exponential decay in the case of only aqueous surfactant solutions.
However, the non-dimensional normalised contact angle ∗ follows almost a linear relation with time and reaches the equilibrium value of contact angle. The study of effect of only particles was limited to only Al2O3 nanoparticles among the particles considered in the present study because the long term stability of other particles (such as CNT and graphene which has been extended in the case of surfactant based colloids) without surfactant is very poor. The implication from the observation is that the solely nanocolloidal suspension’s behaviour is grossly different from that of the solely surfactant solutions and the spreading dynamics in the case of nano colloidal solutions can be a result of mainly two factors: (a) The structural layering at the three phase contact line and the spreading due to the structural component of disjoining pressure. (b) Transients in the surface tension due to the adsorption–desorption dynamics to the liquid-vapour and solid-liquid interfaces and the Marangoni stresses due to local concentration gradients.
The interfacial interaction in the case of nano colloidal suspensions is very complex due to the multiphase zone comprising of interphase of solid nanoparticles, the suspended base fluid and the coexisting solid and the vapour interfaces. The conventional triple phase 11 ACS Paragon Plus Environment
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contact line is thus now converted to a complex quad-phase contact zone. The characteristic energetics of interactions at these nanoscale level interfaces depends on the properties of the nanoparticles and the base fluid. The vector sum of these interfacial forces at the nanoscale collectively determines the localized surface tension of the resulting fluid. The wettability of the particles at the interface is one of the basic driving factors determining the affinity of the particle towards one of the interface27. The energetic difference when such a particle (of radius Rp) moves from the bulk to the fluid–fluid interface in a fluid medium of surface tension γ is expressed as28 ∆$% = − '()* ! (1 + -./
!
(2)
From the thermodynamic aspect, the free energy change due to adsorption of a nanoscale particle is of the order of ~ kBT28 which implies that the particles are susceptible to thermal excitations and are in random motion and consequently in dynamic equilibrium with the bulk suspension. Hence it implies that the particles are in dynamic equilibrium with the interface which essentially affects the spreading dynamics due to the alteration of the interfacial energies. Also this will cause local concentration difference which affects the Marangoni stresses and ultimately will be reflected in the spreading dynamics.
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Figure 3: (a) Illustration of the variation of the base diameter and the dynamic contact angle
( with time for 0.1 percent by weight Al2O3 nanocolloidal solution. (b) Variation of non-
dimensional contact angle ( ∗ with time at different particle concentrations of Al2O3
nanocolloid. (c) Variation of equilibrium time (teq) with the particle concentration. The experimental uncertainty in base diameter was found to be within ± 4% of the spreading diameter and the equilibrium spreading time
The adsorption-desorption dynamics is again itself dependent on the interfaces involved. This essentially implies that a specific nanoparticle in the bulk has an adsorption tendency to the solid-liquid interface and also to the liquid-vapour interface. This apparently leads to a preferential adsorption desorption-characteristics in nanocolloidal solutions subjected to two boundary interfaces namely liquid-air and solid-liquid interface. In the case of surfactant alone, the only mechanism responsible is the interfacial adsorption and the 13 ACS Paragon Plus Environment
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transient surface tension which balances the viscous dissipation. This essentially follows an exponential variation of normalised contact angle. The two mechanisms outlined above are to a certain extent responsible for spreading in the case of nanofluids. But the dominating mechanism is the displacement of the three phase contact line due to the structural component of disjoining pressure as the other mechanism is prevalent in all the systems considered in the present study. Solutions of only nanocolloidal suspensions show almost a linear behaviour in the variation of the contact angle. Also this mechanism is too slow as compared to that of the interfacial diffusion driven adsorption mechanism as the time scale of the teq and rate of change of contact radius variation observed are too small in the case only nanofluids. The slowest process is the rate determining process. As the interfacial adsorption-desorption process is common to both the surfactant case and nanoparticle case, and in only nanoparticle case we are observing a linear variation of the contact angle, it can be elucidated that the rate determining step is the structural component of disjoining pressure and this is the rate determining step in the case of only nanocolloidal solutions. It can be observed that the teq increases initially with particle concentration and suddenly attains a plateau shape thereafter with further increase in particle concentration as illustrated in Fig. 3(c). As described earlier, the dominant mechanism for the spreading in the case of only nanocolloidal suspensions is that of the layering at the precursor film ahead of the apparent three phase contact point. As the particles are added initially it will form the layering at the film and induces the wedge action and results in spreading of the nanofluid solution. But as the bulk particle concentration increases, the near three phase precursor film is arrested by the increased particle population due to plugging. Hence further increase in the bulk particle concentration is not potent enough to bring change to the local concentration of the wedge as it is locally saturated. 3.2.1. Theoretical Scaling Analysis: The spreading of generalized pure fluids is generally expressed in the form of spreading coefficient described by 0 = (12 − (13 − (32
(3)
where (32 represents the liquid-gas interfacial tension, (13 solid-liquid interfacial tension, (12 solid-gas interfacial tension. Nanofluid spreading is enhanced by the structural component of disjoining pressure because of the ordering of nanoparticles in the wedge film and hence
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result in achieving a higher spreading factor. The wedge film contact angle is related to the disjoining pressure via the Frumkin–Derjaguin equation as29:
0 = 4 (ℎ ℎ + 68 4(ℎ 7ℎ = (32 (-./ − 1 9
(4)
where ℎ is the equilibrium film thickness, 4 is the sum of the capillary and hydrostatic
pressure of the droplet and 4 is the disjoining pressure which is sum of three major components as 4 = 4:; + 4 + 41
