Dynamic wetting of a fluoropolymer surface by ionic liquids

On the Dynamic Contact Angle of Capillary-Driven Microflows in Open Channels

The true value of the contact angle between a liquid and a solid is a thorny problem in capillary microfluidics. The Lucas-Washburn-Rideal (LWR) law assumes a constant contact angle during fluid penetration. However, recent experimental studies have shown lower liquid velocities than those predicted by the LWR equation, which are attributed to a velocity-dependent dynamic contact angle that is larger than its static value. Inspection of fluid penetration in closed channels has confirmed that a dynamic angle is needed in the LWR equation. In this work, the dynamic contact angle in an open-channel configuration is investigated using experimental data obtained with a range of liquids, aqueous and organic, and a PMMA substrate. We demonstrate that a dynamic contact angle must be used to explain the early stages of fluid penetration, i.e., at the start of the viscous regime, when flow velocities are sufficiently high. Moreover, the open-channel configuration, with its free surface, enhances the effect of the dynamic contact angle, making its inclusion even more important. We found that for the liquids in our study, the molecular-kinetic theory is the most accurate in predicting the effect of the dynamic contact angle on liquid penetration in open channels.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Dynamic Wetting of Ionic Liquid Drops on Hydrophobic Microstructures

Ionic liquids (ILs)─salts in a liquid state─play a crucial role in various applications, such as green solvents for chemical synthesis and catalysis, lubricants, especially for micro- and nanoelectromechanical systems, and electrolytes in solar cells. These applications critically rely on unique or tunable bulk properties of ionic liquids, such as viscosity, density, and surface tension. Furthermore, their interactions with different solid surfaces of various roughness and structures may uphold other promising applications, such as combustion, cooling, and coating. However, only a few systematic studies of IL wetting and interactions with solid surfaces exist. Here, we experimentally and theoretically investigate the dynamic wetting and contact angles (CA) of water and three kinds of ionic liquid droplets on hydrophobic microstructures of surface roughness (r = 2.61) and packing fraction (ϕ = 0.47) formed by micropillars arranged in a periodic pattern. The results show that, except for water, higher-viscosity ionic liquids have greater advancing and receding contact angles with increasing contact line velocity. Water drops initially form a gas-trapping, CB wetting state, whereas all three ionic liquid drops are in a Wenzel wetting state, where liquids penetrate and completely wet the microstructures. We find that an existing model comparing the global surface energies between a CB and a Wenzel state agrees well with the observed wetting states. In addition, a molecular dynamic model well predicts the experimental data and is used to explain the observed dynamic wetting for the ILs and superhydrophobic substrate. Our results further show that energy dissipation occurs more significantly in the three-phase contact line region than in the liquid bulk.

Molecular interactions of ionic liquids with SiO2 surfaces determined from colloid probe atomic force microscopy

Ionic liquids (ILs) interact strongly with many different types of solid surfaces in a wide range of applications, e.g. lubrication, energy storage and conversion, etc. However, due to the nearly immeasurable large number of potential ILs available, identifying the appropriate ILs for specific solid interfaces with desirable properties is a challenge. Theoretical studies are highly useful for effective development of design and applications of these complex molecular systems. However, obtaining reliable force field models and interaction parameters is highly demanding. In this work, we apply a new methodology by deriving the interaction parameters directly from the experimental data, determined by colloid probe atomic force microscopy (CP-AFM). The reliability of the derived interaction parameters is tested by performing molecular dynamics simulations to calculate translational self-diffusion coefficients and comparing them with those obtained from NMR diffusometry.

Effects of Molecular Chain Length on the Contact Line Movement in Water/n-Alkane/Solid Systems

The movement of the contact line in liquid-liquid-solid systems is a major phenomenon in natural and industrial processes. In particular, n-alkanes are widely occurring in the oil, soil pollution, and chemical industries, yet there is little knowledge on the effects of molecular chain length on the contact line movement. Here, we studied the effects of molecular chain length on the contact line movement in water/n-alkane/solid systems with different surface wettabilities. We used n-heptane (C7), n-decane (C10), and n-hexadecane (C16) as alkanes and α-quartz as the solid surface. We calculated the time-variation contact line moving velocity and also analyzed the jump frequency and the mean distance of the molecular displacement occurring within the contact line zone by molecular-kinetic theory. Molecular dynamics simulation results show that the contact line velocity decreases with increasing the chain length, originally caused by the decreasing the jump frequency and mean distance. These variations with the molecular chain length are related to the more torsions and deformations of the molecules with a longer chain length. In addition, the moving mechanism of the contact line on the same solid surface does not change at different molecular chain lengths, implying that the moving mechanism mainly depends on the three-phase wettability.

Spontaneous Displacement of High Viscosity Micrometer Size Oil Droplets from a Curved Solid in Aqueous Solutions

Spontaneous displacement of high viscosity (∼10³ Pa·s) micrometer size oil droplets from a curved solid in aqueous solutions was investigated. For high viscosity oils, the dynamic droplet shape was found to deviate significantly from a spherical cap shape due to the considerable viscous force in the oil phase. The displacement dynamics of high viscosity droplets were analyzed using molecular kinetic and hydrodynamic models. The molecular kinetic model was found to describe the dynamic displacement well for the droplets of small departure from the spherical cap shape, while the hydrodynamic model is more applicable to the droplets of higher three-phase contact line displacement velocities and hence larger deviation of the droplets from the spherical cap shape.

Moving mechanisms of the three-phase contact line in a water–decane–silica system

The movement of the three-phase contact line with chain molecules in the liquid phase displays more complex mechanisms compared to those in the usual liquid–liquid–solid systems and even to the gas–liquid–solid systems controlled by the traditional single-molecule adsorption–desorption mechanisms. By introducing decane molecules with chain structures, we demonstrate from molecular dynamics insights that the moving mechanism of the contact line in a water–decane–silica system is totally different from traditional mechanisms. Three different wettability-related moving mechanisms including “Roll up”, “Piston” and “Shear” are revealed corresponding to the hydrophilic, intermediate and hydrophobic three-phase wettability, respectively. In the “Roll up” mechanism, the decane molecules are rolled up by the competitively adsorbed water molecules and then move forward under the driving force; when the “Piston” mechanism happens, the decane molecules are pushed by the piston-like water phase owing to the comparable adsorption interactions of the two liquids on the solid surface; in the “Shear” mechanism, the contact line is hard to drive due to the stronger decane–silica interactions but the decane molecules far away from the solid surface will move forward. Besides, the time-averaged velocity of the moving contact line is greatly related to the moving mechanisms. For the “Roll up” mechanism, the contact line velocity increases first and then reaches a steady value; for the “Piston” mechanism, the contact line velocity has a maximum value at the start-up stage and then decreases to a stable value; for the “Shear” mechanism, the contact line velocity fluctuates around zero due to the thermal fluctuation of the molecules. Additionally, the mean distance from Molecular Kinetics Theory increases with decreasing hydrophilicity and the displacement frequency in “Roll up” mechanism is 2 orders of magnitude higher than that in the “Piston” mechanism, further demonstrating the different moving mechanisms from a quantitative point of view.

Wettability-related moving mechanisms of the three-phase contact line with one liquid phase composed of chain molecules are revealed.

The mechanism and universal scaling law of the contact line friction for the Cassie-state droplets on nanostructured ultrahydrophobic surfaces

Besides the Wenzel state, liquid droplets on micro/nanostructured surfaces can stay in the Cassie state and consequently exhibit intriguing characteristics such as a large contact angle, small contact angle hysteresis and exceptional mobility. Here we report molecular dynamics (MD) simulations of the wetting dynamics of Cassie-state water droplets on nanostructured ultrahydrophobic surfaces with an emphasis on the genesis of the contact line friction (CLF). From an ab initio perspective, CLF can be ascribed to the collective effect of solid-liquid retarding and viscous damping. Solid-liquid retarding is related to the work of adhesion, whereas viscous damping arises from the viscous force exerted on the liquid molecules within the three-phase (liquid/vapor/solid) contact zone. In this work, a universal scaling law is derived to generalize the CLF on nanostructured ultrahydrophobic surfaces. With the decreasing fraction of solid-liquid contact (i.e., the solid fraction), CLF for a Cassie-state droplet gets enhanced due to the fact that viscous damping is counter-intuitively intensified while solid-liquid retarding remains unchanged. Nevertheless, the overall friction between a Cassie-state droplet and the structured surface is indeed reduced since the air cushion formed in the interstices of the surface roughness underneath the Cassie-state droplet applies negligible resistance to the contact line. Our results have revealed the genesis of CLF from an ab initio perspective, demonstrated the effects of surface structures on a moving contact line and justified the critical role of CLF in the analysis of wetting-related situations.

Dynamic wetting of imidazolium-based ionic liquids on gold and glass

Experimental data and theoretical fitting for ionic liquid wetting on application-relevant substrates.

Interfacial Nanostructure and Asymmetric Electrowetting of Ionic Liquids

In this work, the interfacial nanostructure and electrowetting of ionic liquids having the same 1-ethyl-3-methylimidazolium cation ([EMIm]⁺) but different anions such as bis(trifluoromethylsulfonyl)imide (TFSI^-), trifluoromethylsulfonate (TfO^-), methylsulfonate (OMs^-), acetate (OAc^-), bis(fluorosulfonyl)imide (FSI^-), dicyanamide (DCA^-), and tris(pentafluorethyl)trifluorphosphat (FAP^-) on bare metallic electrodes were investigated. In the investigated voltammetric potential regime, the contact angle versus voltage curve is asymmetric with respect to surface polarity. The electrowetting of the ILs occurs at negative potentials but does not occur at positive potentials. In situ atomic force microscopy (AFM) shows that the IL adopts a multilayered structure at the solid/IL interface, and a cation-rich layer is present in the innermost layer during cathodic polarization. The cations can change their orientation and propagate ahead of the three-phase contact line by diffusion, leading to further spreading on the negatively charged surface. The formation of such a surface layer is also evidenced by X-ray photoelectron spectroscopy. Such a surface diffusion mechanism does not occur during anodic polarization, where anions are enriched. In addition, the influence of substrate, water, and dissolved zinc salts on the electrowetting of ILs was studied. Our findings provide valuable insights for the interfacial nanostructure and the electrowetting of ILs.

Analyzing the Molecular Kinetics of Water Spreading on Hydrophobic Surfaces via Molecular Dynamics Simulation

In this paper, we report molecular kinetic analyses of water spreading on hydrophobic surfaces via molecular dynamics simulation. The hydrophobic surfaces are composed of amorphous polytetrafluoroethylene (PTFE) with a static contact angle of ~112.4° for water. On the basis of the molecular kinetic theory (MKT), the influences of both viscous damping and solid-liquid retarding were analyzed in evaluating contact line friction, which characterizes the frictional force on the contact line. The unit displacement length on PTFE was estimated to be ~0.621 nm and is ~4 times as long as the bond length of C-C backbone. The static friction coefficient was found to be ~\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${10}^{-3}$$\end{document}10−3 Pa·s, which is on the same order of magnitude as the dynamic viscosity of water, and increases with the droplet size. A nondimensional number defined by the ratio of the standard deviation of wetting velocity to the characteristic wetting velocity was put forward to signify the strength of the inherent contact line fluctuation and unveil the mechanism of enhanced energy dissipation in nanoscale, whereas such effect would become insignificant in macroscale. Moreover, regarding a liquid droplet on hydrophobic or superhydrophobic surfaces, an approximate solution to the base radius development was derived by an asymptotic expansion approach.

Spreading Dynamics of Molten Polymer Drops on Glass Substrates

Wetting dynamics drive numerous processes involving liquids in contact with solid substrates with a wide range of geometries. The spreading dynamics of organic liquids and liquid metals at, respectively, room temperature and >1000 °C have been studied extensively, both experimentally and numerically; however, almost no attention has been paid to the wetting behavior of molten drops of thermoplastic polymers, despite its importance, for example, in the processing of fiber-reinforced polymer composites. Indeed, the ability of classical theories of dynamic wetting, that is, the hydrodynamic and the molecular-kinetic theories, to model these complex liquids is unknown. We have therefore investigated the spreading dynamics on glass, over temperatures between 200 and 260 °C, of two thermoplastics: polypropylene (PP) and poly(vinylidene fluoride) (PVDF). PP and PVDF showed, respectively, the highest and lowest slip lengths due to their different interactions with the glass substrate. The jump lengths of PP and PVDF are comparable to their Kuhn segment lengths, suggesting that the wetting process of these polymers is mediated by segmental displacements. The present work not only provides evidence of the suitability of the classical models to model dynamic wetting of molten polymers but also advances our understanding of the wetting dynamics of molten thermoplastics at the liquid/solid interface.

A smart surface with switchable wettability by an ionic liquid

Smart control of surface wettability by ionic liquids (ILs) is significant for designing IL-related intelligent materials and devices. Herein, we present mixed molecular brushes comprised of poly(phenylethyl methacrylate) and 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PPhEtMA-co-PFDMS) grafted surfaces that are capable of dynamically regulating 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf₂]) wettability. ¹H NMR and quartz crystal microbalance characterization demonstrate that the wettability changes result from a temperature-dominated cation-π interaction between [EMIm][NTf₂] and PPhEtMA-co-PFDMS brushes.

Addition of low concentrations of an ionic liquid to a base oil reduces friction over multiple length scales: a combined nano- and macrotribology investigation

The efficacy of ionic liquids (ILs) as lubricant additives to a model base oil has been probed at the nanoscale and macroscale as a function of IL concentration using the same materials.

Contact angles and wettability of ionic liquids on polar and non-polar surfaces

Many applications involving ionic liquids (ILs) require the knowledge of their interfacial behaviour, such as wettability and adhesion. In this context, herein, two approaches were combined aiming at understanding the impact of the IL chemical structures on their wettability on both polar and non-polar surfaces, namely: (i) the experimental determination of the contact angles of a broad range of ILs (covering a wide number of anions of variable polarity, cations, and cation alkyl side chain lengths) on polar and non-polar solid substrates (glass, Al-plate, and poly-(tetrafluoroethylene) (PTFE)); and (ii) the correlation of the experimental contact angles with the cation-anion pair interaction energies generated by the Conductor-like Screening Model for Real Solvents (COSMO-RS). The combined results reveal that the hydrogen-bond basicity of ILs, and thus the IL anion, plays a major role through their wettability on both polar and non-polar surfaces. The increase of the IL hydrogen-bond accepting ability leads to an improved wettability of more polar surfaces (lower contact angles) while the opposite trend is observed on non-polar surfaces. The cation nature and alkyl side chain lengths have however a smaller impact on the wetting ability of ILs. Linear correlations were found between the experimental contact angles and the cation-anion hydrogen-bonding and cation ring energies, estimated using COSMO-RS, suggesting that these features primarily control the wetting ability of ILs. Furthermore, two-descriptor correlations are proposed here to predict the contact angles of a wide variety of ILs on glass, Al-plate, and PTFE surfaces. A new extended list is provided for the contact angles of ILs on three surfaces, which can be used as a priori information to choose appropriate ILs before a given application.

Static and dynamic wetting behaviour of ionic liquids

Ionic liquids (ILs) are a unique family of molecular liquids ('molten salts') that consist of a combination of bulky organic cations coupled to inorganic or organic anions. The net result of steric hindrance and strong hydrogen bonding between components results in a material that is liquid at room temperature. One can alter the properties of ionic liquids through chemical modification of anion and cation, thus tailoring the IL for a given application. One such property that can be controlled or selected is the wettability of an IL on a particular solid substrate. However, the study of wetting of ionic liquids is complicated by the care required for accurate and reproducible measurement, due to both the susceptibility of the IL properties to water content, as well as to the sensitivity of wettability measurements to the state of the solid surface. This review deals with wetting studies of ILs to date, including both static and dynamic wetting, as well as issues concerning line tension and the formation of precursor and wetting films.

The molecular-kinetic approach to wetting dynamics: Achievements and limitations

The molecular-kinetic theory (MKT) of dynamic wetting was formulated almost 50 years ago. It explains the dependence of the dynamic contact angle on the speed of a moving meniscus by estimating the non-hydrodynamic dissipation in the contact line. Over the years it has been refined to account explicitly for the influence of (bulk) fluid viscosity and it has been applied successfully to both solid-liquid-vapour and solid-liquid-liquid systems. The free energy barrier for surface diffusion has been related to the energy of adhesion. The MKT provides a qualitative explanation for most effects in dynamic wetting. The theory is simple, flexible, and it is widely used to rationalize the physics of wetting dynamics and fit experimental data (dynamic contact angle versus contact line speed). The MKT predicts an intermediate wettability as optimal for high-speed coating as well as the maximum speeds of wetting and dewetting. Nevertheless, the values of the molecular parameters derived from experimental data tend to be scattered and not particularly reliable. This review outlines the main achievements and limitations of the MKT and highlights some common cases of misinterpretation.

The influence of topography on dynamic wetting

The paramount importance of wetting applications and the significant economic value of controlling wetting-based industrial processes has stimulated a deep interest in wetting science. In many industrial applications the motion of a complex liquid front over nano-textured surfaces controls the fate of the processes. However our knowledge of the impact of nano-heterogeneities on static and dynamic wetting is very limited. In this article, the fundamentals of wetting are briefly reviewed, with a particular focus on hysteresis and roughness issues. Present knowledge and models of dynamic wetting on smooth and rough surfaces are then examined, with particular attention devoted to the case of nano-topographical heterogeneities and solid-fluid-fluid systems.

Impact of nanoscale surface heterogeneity on precursor film growth and macroscopic spreading of [Rmim][NTf2] ionic liquids on mica

The connection between the interfacial properties of ionic liquids and their wetting behavior has been studied very little to date and not at all on heterogeneous surfaces. Therefore, we have investigated the static and dynamic wetting for a family of ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], on mica, where R represents an ethyl, butyl, or hexyl alkyl chain on the imidazolium ring. Spreading is impacted greatly by a precursor film that forms on both homogeneous and heterogeneous mica surfaces. Macroscopically, the initial viscous spreading of the ionic liquid droplet on bare mica occurs within seconds but is then followed by a very slow relaxation that can be closely correlated with the typical time-scales of the precursor film growth. The contact angle for [emim][NTf2] and [bmim][NTf2] relaxes from about 40° to 23° over 30 and 90 min, respectively. For [hmim][NTf2], the process takes approximately 24 h and approaches complete wetting. The thickness of the precursor films for [emim][NTf2], [bmim][NTf2], and [hmim][NTf2] were 0.53, 0.65, and 1.0 nm, respectively, according to atomic force microscopy (AFM). These values are consistent with a monolayer of ionic liquid cations on mica, rather than ion pairs. A monolayer of octadecylphosphonic acid (OPA) on mica prevents both the formation of a precursor film and the relaxation of the contact angle. However, only a partial surface coverage of ~60% OPA is required to have the same effect. Quenching of precursor film formation (and associated contact angle relaxation) is due to an increasingly connected network of OPA regions that closes the nanoscale paths of bare mica on which the precursor film can develop via surface diffusion.

Toward a predictive theory of wetting dynamics

The molecular kinetic theory (MKT) of dynamic wetting, first proposed nearly 50 years ago, has since been refined to account explicitly for the effects of viscosity and solid-liquid interactions. The MKT asserts that the systematic deviation of the dynamic contact angle from its equilibrium value quantitatively reflects local energy dissipation (friction) at the moving contact line as it traverses sites of solid-liquid interaction. Specifically, it predicts that the coefficient of contact-line friction ζ will be proportional to the viscosity of the liquid ηL and exponentially dependent upon the strength of solid-liquid interactions as measured by the equilibrium work of adhesion Wa(0). Here, we analyze a very large set of dynamic wetting data drawn from more than 20 publications and representative of a very wide range of systems, from molecular-dynamics-simulated Lenard-Jones liquids and substrates, through conventional liquids and solids, to molten glasses and liquid metals on refractory solids. The combined set spans 9 decades of viscosity and 11 decades of contact-line friction. Our analysis confirms the predicted dependence of ζ upon ηL and Wa(0), although the data are scattered. In particular, a plot of ln(ζ/ηL) versus Wa(0)/n (i.e., the work of adhesion per solid-liquid interaction site) is broadly linear, with 85% of the data falling within a triangular envelope defined by Wa(0) and 0.25Wa(0). Various reasons for this divergence are explored, and a semi-empirical approach is proposed to predict ζ. We suggest that the broad agreement between the MKT and such a wide range of data is strong evidence that the local microscopic contact angle is directly dependent upon the velocity of the contact line.

Dynamic electrowetting and dewetting of ionic liquids at a hydrophobic solid-liquid interface

The dynamic electrowetting and dewetting of ionic liquids are investigated with high-speed video microscopy. Five imidazolium-based ionic liquids ([BMIM]BF(4), [BMIM]PF(6), [BMIM]NTf(2), [HMIM]NTf(2), and [OMIM]BF(4)) are used as probe liquids. Droplets of ionic liquids are first spread on an insulated electrode by applying an external voltage (electrowetting) and then allowed to retract (dewetting) when the voltage is switched off. The base area of the droplet varies exponentially during both the electrowetting and retraction processes. The characteristic time increases with the viscosity of the ionic liquid. The electrowetting and retraction kinetics (dynamic contact angle vs contact line speed) can be described by the hydrodynamic or the molecular-kinetic model. Energy dissipation occurs by viscous and molecular routes with a larger proportion of energy dissipated at the three-phase contact line when the liquid meniscus retracts from the solid surface. The outcomes from this research have implications for the design and control of electro-optical imaging systems, microfluidics, and fuel cells.

Forced wetting of a reactive surface

The dynamic wetting of water on gelatin-coated poly(ethylene terephthalate) (GC-PET) has been investigated by forced wetting over a wide speed range and compared with earlier data obtained with unmodified PET. The results were analysed according to the molecular-kinetic theory of dynamic wetting (MKT). Both substrates show complex behaviour, with separate low- and high-speed modes. For the GC-PET, this is attributed to a rapid change in the wettability of the substrate on contact with water, specifically a surface molecular transformation from hydrophobic to hydrophilic. This results in a smooth wetting transition from one mode to the other. For the PET, the bimodal behaviour is attributed to surface heterogeneity, with the low-speed dynamics dominated by interactions with polar sites on the substrate that become masked at higher speeds. In this case, the transition is discontinuous. The study has general ramifications for the investigation of any wetting processes in which a physicochemical transformation takes place at the solid surface on contact with the liquid. In particular, it shows how forced wetting, combined with the MKT, can reveal subtle details of the processes involved. It is unlikely that similar insight could be gained from spontaneous wetting studies, such as spreading drops.

Predicting the wetting dynamics of a two-liquid system

We propose a new theoretical model of dynamic wetting for systems comprising two immiscible liquids, in which one liquid displaces another from the surface of a solid. Such systems are important in many industrial processes and the natural world. The new model is an extension of the molecular-kinetic theory of wetting and offers a way to predict the dynamics of a two-liquid system from the individual wetting dynamics of its parent liquids. We also present the results of large-scale molecular dynamics simulations for one- and two-liquid systems and show them to be in good agreement with the new model. Finally, we show that the new model is consistent with the limited data currently available from experiment.

Experimental evidence of the role of viscosity in the molecular kinetic theory of dynamic wetting

We report an experimental study of the dynamics of spontaneous spreading of aqueous glycerol drops on glass. For a range of glycerol concentrations, we follow the evolution of the radius and contact angle over several decades of time and investigate the influence of solution viscosity. The application of the molecular kinetic theory to the resulting data allows us to extract the coefficient of contact-line friction ζ, the molecular jump frequency κ(0), and the jump length λ for each solution. Our results show that the modified theory, which explicitly accounts for the effect of viscosity, can successfully be applied to droplet spreading. The viscosity affects the jump frequency but not the jump length. In combining these data, we confirm that the contact-line friction of the solution/air interface against the glass is proportional to the viscosity and exponentially dependent on the work of adhesion.