Forced capillary wetting of viscoelastic fluids

HYPOTHESIS

Achieving rapid capillary wetting is highly desirable in nature and industries. Previous endeavors have primarily concentrated on passive wetting strategies through surface engineering. However, these approaches are inadequate for high-viscosity fluids due to the significant viscous resistance, especially for non-Newtonian fluids. In contrast, forced wetting emerges as a promising method to address the challenges associated with achieving rapid wetting of non-Newtonian fluids in capillaries.

EXPERIMENTS

To investigate the forced wetting behavior of viscoelastic fluids in capillaries, we employ Xanthan Gum (XG) aqueous solutions as target fluids with the storage modulus significantly exceeding the loss modulus. We utilize smooth glass capillaries connected to a syringe pump to achieve high moving speeds of up to 1 m/s.

FINDINGS

Our experiments reveal a significant distinction in the power-law exponent that governs the scaling relationship between the dynamic contact angle and velocity for viscoelastic fluids compared to Newtonian fluids. This exponent is considerably smaller and varies based on the concentration of viscoelastic fluids and the diameter of the capillaries. We suggest that the viscosity dominates the wetting dynamics of viscoelastic fluids, manifested by the contact line morphology-dependent behavior. This insight has significant implications for microfluidics and drug injectability.

Dynamic wetting of Newtonian and viscoelastic fluids on microstructured surfaces

Hypothesis Although extensive research has been conducted on the dynamic wetting of Newtonian fluids, limited insights have been gained for viscoelastic fluids, particularly on engineered surfaces. We hypothesize that differences in dynamic wetting on microstructured surfaces exist between such fluids, which may be attributed to variations in viscosity and elasticity as well as changes in the microscopic morphology of the moving contact line. Experiments To systematically investigate the wetting differences between Newtonian and viscoelastic fluids on microstructured surfaces, we conducted forced wetting experiments of glycerol-water and carboxymethyl cellulose aqueous solutions on microstructured polytetrafluoroethylene surfaces through a modified Wilhelmy plate method. Findings Results demonstrated an apparent difference in the relationship between the dynamic contact angle and moving velocity with different microstructured surfaces for Newtonian and viscoelastic fluids. The power-law exponent between the capillary number and cubic of the dynamic contact angle increases with the strengthening of shear thinning and elastic effects. In contrast, this exponent is rarely influenced by the scale of microstructured surfaces, particularly in highly viscous regions where viscous force dominates. In addition, viscosity affects the viscous bending and distance that liquid molecules jump at the contact line. These findings have potential applications in coating complex fluids on engineered surfaces.

How tank-mix adjuvant type and concentration influence the contact angle on wheat leaf surface

Currently, the utilization of unmanned aerial vehicles (UAVs) for spraying pesticides is a prevalent issue in Asian countries. Improving the pesticide efficiency of UAV spraying is a major challenge for researchers. One of the factors that affect the efficiency is the wetting property of the spraying solutions on crop leaves. Tank-mix adjuvants, which can modify the wetting ability of the solutions, are often used for foliar application. However, different types and concentrations of tank-mix adjuvants may have different impacts on the wetting properties of droplets. In this article, we investigated the effects of four tank-mix adjuvants, Beidatong (BDT), Velezia Pro (VP), Nongjianfei (NJF), and Lieying (LY), on the dynamic contact angle (CA) values of droplets on the adaxial surface of wheat leaves. We measured the dynamic CA values of various concentrations of each adjuvant solution and determined the optimal concentrations based on the CA values, droplet spreading time, and cost. The results showed that adding any of the four adjuvants decreased the CA values, but the patterns of decrease varied among them. The CAs of BDT and VP solutions decreased slowly during the observation time (0-8.13 s), while those of NJF and LY solutions decreased rapidly throughout the observation period. According to the dynamic CA values of different concentrations, the optimal concentrations of BDT, VP, NJF, and LY for wheat field application were 12%, 16%, 6‰, and 0.3‰, respectively. Alkoxy-modified polytrisiloxane adjuvant (LY) could be recommended as an appropriate tank-mix adjuvant for wheat field application, considering spreading efficiency and cost. This study provides theoretical and practical guidance for selecting and optimizing tank-mix adjuvants for UAV spraying.

A generalized scaling theory for spontaneous spreading of Newtonian fluids on solid substrates

HYPOTHESIS

There exists a generalized solution for the spontaneous spreading dynamics of droplets taking into account the influence of interfacial tension and gravity.

EXPERIMENTS

This work presents a generalized scaling theory for the problem of spontaneous dynamic spreading of Newtonian fluids on a flat substrate using experimental analysis and numerical simulations. More specifically, we first validate and modify a dynamic contact angle model to accurately describe the dependency of contact angle on the contact line velocity, which is generalized by the capillary number. The dynamic contact model is implemented into a two-phase moving mesh computational fluid dynamics (CFD) model, which is validated using experimental results.

FINDINGS

We show that the spreading process is governed by three important parameters: the Bo number, viscous timescale τ_viscous, and static advancing contact angle, θ_s. More specifically, there exists a master spreading curve for a specific Bo and θ_s by scaling the spreading time with the τ_viscous. Moreover, we developed a correlation for prediction of the equilibrium shape of the droplets as a function of both Bo and θ_s. The results of this study can be used in a wide range of applications to predict both dynamic and equilibrium shape of droplets, such as in droplet-based additive manufacturing.

Dynamic contact angle measurements on lubricant infused surfaces

HYPOTHESIS

Even though lubricant-infused surfaces (LISs) are known to affect the mobility of working fluid depending on the infused lubricant, previous studies have not yet quantified their slippery property. This study proposes the slippery nature of the LIS can be assessed by dynamic contact angles of the working fluid on the LIS and its scaling model.

EXPERIMENTS

We measured the apparent dynamic advancing and receding contact angles on a LIS using a modified Wilhelmy plate technique for the first time. Lubricant having different viscosities was infused into the sanded polytetrafluoroethylene surface to fabricate the LIS. The surface was immersed into or withdrawn from an aqueous glycerol-water solution by varying the capillary number and the lubricant viscosity.

FINDINGS

The dynamic contact angles on LIS was found to be sensitive to changes in both the lubricant viscosity and the capillary number. The cube of the dynamic contact angles on the LIS was proportional to θ_D³~Ca¹, which follows a conventional hydrodynamic theory. In addition, the decreasing lubricant viscosity shifted the cube of the dynamic contact angles to high capillary numbers. Our dynamic contact angle data coincided with the prediction from a scaling law derived in this study.

A linear regime of hysteresis for calculating the dynamic contact angle under low capillary numbers with displacement experiments in microscale PDMS microchannels

HYPOTHESIS

Determining capillary pressures and permeation resistances of multiphase seepage in porous-type reservoirs is crucial. The dynamic contact angle (CA) is one of the critical parameters used to calculate capillary pressures and permeation resistances. Under reservoir conditions, dynamic CAs are rate-independent of the triple-phase contact line velocity. Calculating rate-independent dynamic CAs remains challenging and is the main focus of this paper.

EXPERIMENTS

An experimental system was designed to capture the dynamic CAs of liquid-fluid displacements in microscale polydimethylsiloxane (PDMS) microchannels. These microchannels were 20 × 80 μm or 20 × 40 μm in height and width. The capillary numbers (Ca) were controlled to satisfy the conditions of Ca < 4 × 10^-5 for liquid-air displacements and Ca < 3.5 × 10^-7 for liquid-liquid displacements.

FINDINGS

Since pinning forces were consistent at the receding interface and the advancing interface, energy barriers exhibited symmetry. However, CA hysteresis exhibited asymmetry. Based on our experiments, a linear regime of hysteresis was developed and verified to be consistent with others' experiments. The relation can be used to calculate hysteresis strengths and rate-independent CAs. This model was also compared with the fractal model. The hysteresis factor in this model can be derived using the Wenzel factor r and the Cassie fraction ϕ_s.

Wetting dynamics and surface energy components of single carbon fibers

HYPOTHESIS

Measuring contact angles made by liquids around individual carbon fibers (CFs) using the Wilhelmy technique is a conventional method to evaluate their surface properties. However, despite its apparent simplicity, inaccurate measurements of capillary forces and wetted lengths, due to the fineness of the CFs, as well as an improper selection of probe liquids can lead to incorrect contact angle and surface energy calculations, leading to an erroneous characterization of their surface properties.

EXPERIMENTS

In this study, dynamic wetting experiments of individual CFs were performed in ethylene glycol, diiodomethane, and formamide based on the Wilhelmy method. Capillary forces exerted on the CFs were recorded and analyzed in detail to calculate reliable dynamic contact angles at different contact-line velocities. The molecular-kinetic theory (MKT) and hydrodynamic approach (HD) were then used to model the experimental data and to obtain static contact angles.

FINDINGS

The analysis shows that the experimental data are in good agreement with the linear MKT suggesting that the dominant channel of energy dissipation at the contact line is the contact-line friction. From the predicted static advancing contact angle values, the surface energy components of the CFs could be obtained thus providing a way to characterize their interfacial properties and predict their compatibility with polymer matrices. This study furthermore points out the importance of choosing the correct combination of test fluids to obtain reliable surface energy results.

Capillary rise of polydimethylsiloxane around a poly(ethylene terephthalate) fiber versus viscosity: Existence of a sharp transition in the dynamic wetting behavior

HYPOTHESIS

Since the emergence of the molecular-kinetic theory and the hydrodynamic approach, it is generally accepted that the displacement of the contact line is controlled by the viscous or frictional channel of energy dissipation for respectively high-viscosity and low-viscosity liquids. However, how the dissipation switches from one channel to another is still unknown. We therefore hypothesized that, by progressively changing the viscosity of a liquid, a better understanding of the underlying mechanism driving this wetting dynamic transition would be obtained.

EXPERIMENTS

Performing capillary rise experiments of polydimethylsiloxane on a poly(ethylene terephthalate) fiber at different temperatures, i.e. at different liquid viscosities, we characterized the transition between the viscous and frictional regimes. The fiber surface topography was also characterized and its effect on the wetting dynamics was quantified.

FINDINGS

The wetting dynamics switched from one regime to the other in a very short viscosity interval. Besides, the wetting behavior in the transition region is sensitive to the fiber surface topography. The presence or the absence of a liquid rim ahead of the contact line actually determines the dominant channel of dissipation.

Wetting dynamics of polydimethylsiloxane mixtures on a poly(ethylene terephthalate) fiber

HYPOTHESIS

The wetting dynamics of liquids with identical surface tensions are mostly controlled by their viscosities. We therefore hypothesized that the wetting dynamics of one- (pure liquid) and two-component (mixture) polydimethylsiloxane (PDMS) on a poly(ethylene terephthalate) (PET) fiber with similar surface tensions and viscosities should be controlled by the same underlying physical mechanisms.

EXPERIMENTS

We studied the capillary rise of PDMS liquids on a PET fiber. We compared the different contact angle relaxations and characterized the transitions between the molecular-kinetic theory (MKT) and hydrodynamic approach (HD) for the PDMS mixtures and the pure liquids as a function of their viscosities.

FINDINGS

Compared to the pure PDMS liquid with a viscosity of 20 mm²/s that presents a contact angle relaxation following a t^-1/2 scale law in agreement with HD, the PDMS mixture with a higher viscosity (27.4 mm²/s) shows a t^-1 behavior predicted by the MKT. Moreover, the transition between MKT and HD appears in a regime with higher viscosities for PDMS mixtures than for pure liquids. Surface segregation of shorter PDMS chains or precursor film may be responsible for this shift.

Capillary dynamics driven by molecular self-layering

Capillary dynamics is a ubiquitous everyday phenomenon. It has practical applications in diverse fields, including ink-jet printing, lab-on-a-chip, biotechnology, and coating. Understanding capillary dynamics requires essential knowledge on the molecular level of how fluid molecules interact with a solid substrate (the wall). Recent studies conducted with the surface force apparatus (SFA), atomic force microscope (AFM), and statistical mechanics simulation revealed that molecules/nanoparticles confined into the film/wall surfaces tend to self-layer into 2D layer/s and even 2D in-layer with increased confinement and fluid volume fraction. Here, the capillary rise dynamics of simple molecular fluids in cylindrical capillary is explained by the molecular self-layering model. The proposed model considers the role of the molecular shape on self-layering and its effect on the molecularly thin film viscosity in regards to the advancing (dynamic) contact angle. The model was tested to explain the capillary rise dynamics of fluids of spherical, cylindrical, and disk shape molecules in borosilicate glass capillaries. The good agreement between the capillary rise data and SFA data from the literature for simple fluid self-layering shows the validity of the present model. The present model provides new insights into the design of many applications where dynamic wetting is important because it reveals the significant impact of molecular self-layering close to the wall on dynamic wetting.

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.

Impact of air and water vapor environments on the hydrophobicity of surfaces

HYPOTHESIS

Droplet wettability and mobility play an important role in dropwise condensation heat transfer. Heat exchangers and heat pipes operate at liquid-vapor saturation. We hypothesize that the wetting behavior of liquid water on microstructures surrounded by pure water vapor differs from that for water droplets in air.

EXPERIMENTS

The static and dynamic contact angles and contact angle hysteresis of water droplets were measured in air and pure water vapor environments inside a pressure vessel. Pressures ranged from 60 to 1000 mbar, with corresponding saturation temperatures between 36 and 100°C. The wetting behavior was studied on four hydrophobic surfaces: flat Teflon-coated, micropillars, micro-scale meshes, and nanoparticle-coated with hierarchical micro- and nanoscale roughness.

FINDINGS

Static advancing contact angles are 9° lower in the water vapor environment than in air on a flat surface. One explanation for this reduction in contact angles is water vapor adsorption to the Teflon. On microstructured surfaces, the vapor environment has little effect on the static contact angles. In all cases, variations in pressure and temperature do not influence the wettability and mobility of the water droplets. In most cases, advancing contact angles increase and contact angle hysteresis decreases when the droplets are sliding or rolling down an inclined surface.

VOF simulations of the contact angle dynamics during the drop spreading: standard models and a new wetting force model

INTRODUCTION

In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas-liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.

METHODOLOGY

The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an "adhesion-like" force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid-air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.

RESULTS

The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θadv=10°-70°) and hydrophobic (θadv=105°-120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.

CONCLUSION

It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We<˜80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading.

Capillary rise dynamics of aqueous glycerol solutions in glass capillaries: a critical examination of the Washburn equation

The classic description of capillary rise given by the Washburn equation was recently questioned in the light of experimental evidence for a velocity dependent dynamic contact angle at a moving contact line. We present a systematic investigation of the capillary rise dynamics of glycerol and aqueous glycerol solutions in vertical glass capillaries of various radii. For pure glycerol, the results of our experiments are in almost perfect agreement with the predictions of the Washburn equation using independently measured values for the liquid and capillary parameters. For aqueous glycerol solutions we observe discrepancies between the theoretical expectations and the experimental results, which are increasing with the water content of the solution. A thorough analysis, combined with scaling arguments, allows us to conclude that dynamic contact angle effects alone cannot provide a consistent explanation for these discrepancies. Rather, they can be perfectly accounted for if the mixture flowing in the capillary would have an effective, increased viscosity (in respect to the nominal value). We suggest and briefly discuss various mechanisms that could contribute to this observed behavior.