Constant Contact Line Research Articles

Exploring the impact of pillar edge effects on water drop evaporation

A water drop placed on a small pillar unveils an edge effect, which amplifies volume accumulation within the same contact area. This effect stems from an increased apparent contact angle, resembling hydrophobic surfaces. This emphasizes the influence of surface geometry beyond chemical properties.This study explores the characteristics of the drop by dissecting the interplay between surface forces, geometric attributes, and the evaporation process, enabling a comprehensive understanding of their combined impact on drop behavior.Two distinct evaporation stages were identified: the first stage, characterized by a constant contact line along the pillar edge, and the second stage, involving the detachment of edges and shape alterations influenced by surface roughness.The evaporation rate from the pillar is determined by applying the D2 law and empirical observations. Moreover, the interplay among the key parameters in the process—namely, the evaporation rate, contact angle, volume, and pillar diameter—has been elucidated.

Study on a grinding force model of a variable grinding contact area during knife-edge surface grinding

Abstract. ​​​​​​​In the context of non-standard blade geometries of knife-like tools with tapered cutting edges where the width of the blade surface varies with feed, there is limited research on predicting grinding forces considering the changing contact line. To enhance the accuracy of predicting grinding forces during the blade surface grinding of knife-like tools, a novel analytical-regression correction method is proposed. This method employs an analytical approach to analyze the varying contact line between the grinding wheel and the tool during grinding, enabling the determination of irregularly shaped grinding contact zones. By introducing exponential coefficients related to the grinding contact line, a regression analysis is employed to refine a variable edge-width grinding force model. In comparison to the conventional constant contact line blade surface grinding force prediction, this model is better suited for non-standard blade geometries of knife-like tools in grinding processes. Results indicate that the average relative error between the predicted values from the variable edge-width grinding force model and the actual measurements remains within 9 %, thereby validating the model's effectiveness in predicting grinding forces.

Droplet Evaporation Dynamics on Hydrophobic Network Surfaces.

Surface modification, such as hydrophobic network modification, is very promising technology to control droplet dynamics, heat transfer, and evaporation. However, fundamental mechanisms of how these chemically patterned surfaces affect the droplet evaporation dynamics and predictions of evaporation rates are still lacking. In the present work, we systematically investigated the full process of droplet evaporation dynamics on hydrophobic network surfaces and distinguished four different stages: constant contact line (CCL) stage, constant contact angle (CCA) stage, pattern-pinning (PP) stage, and moving contact line (MCL) stage. We further developed a general model considering the pinning and depinning forces to accurately predict the evaporation transition from PP to MCL stages (i.e., critical receding contact angle, θcr). As for the influence of the chemically patterned surface on the evaporation rate, a corrected contact line length was considered and combined with the well-known Rowan and Erbil's models. Finally, a general model was thus proposed and showed successful predictions for the evaporation durations of each stage.

Janus micro‐thread to micro‐nanodroplets using dynamic contact line lithography

AbstractDynamic contact line lithography (DCLL) is generally used for the template‐less but well‐ordered deposition of polymer micro/nanostructures from the solution of homopolymer. DCLL from the blend of several polymers in a common suitable solvent can generate rich morphologies including nanostructured Janus micro threads on a surface which might be challenging to fabricate using any other method. A blend of polystyrene (PS) and poly(methyl methacrylate) (PMMA) in toluene with different compositions and concentrations are used for DCLL at a constant contact line speed of 10 μm/s. Variations of compositions and the overall concentration of polymer, engender Janus micro threads to undulated threads and multi‐layered micro/nanodroplets of PS/PMMA. Interestingly, the more soluble PS phase separates first compared to less soluble PMMA near the contact line due to the enhanced local concentration of PS during the convective flow of the solvent toward the contact line. This is also verified by DCLL using PS/PMMA blend in ethyl acetate as well. In that case, more soluble PMMA deposits first near the contact line compared to marginally less soluble PS in the solvent.

Simulations of contact angle hysteresis effects in droplet sliding on inclined isothermal surfaces and droplet evaporating on heated horizontal surfaces by a lattice Boltzmann method with a variable solid-fluid interaction strength scheme

A variable solid-fluid interaction strength scheme compatible with the lattice Boltzmann method is introduced to investigate contact angle hysteresis phenomena numerically. This method is applied to study two problems: (i) droplet sliding on inclined isothermal plates and (ii) droplet evaporating on horizontal heated plates. It is demonstrated that this method is capable of reproducing the decrease in the receding contact angle and the increase in the advancing contact angle during droplet sliding on an inclined isothermal plate, and the constant contact line (CCL) and constant contact angle (CCA) evaporation periods during droplet evaporation on a horizontal heated plate. These simulated results are in agreement with previous experimental investigations.

Linking nonisothermal interfacial temperature and flow field measurements at an evaporating droplet

Regular thermal patterns can be formed spontaneously at the evaporating interface of a sessile droplet. Our experimental investigations through the thermography and particle image velocimetry reveal the linkage of the nonuniform interfacial temperature and the flow field in an ethanol droplet which is designed to evaporate on a heated substrate with a constant contact line mode before the last phase of drying. It is suggested that the Bénard-Marangoni instability is responsible for the regular thermal patterns. For the present cases with a fixed substrate temperature, the evolution of deformed Bénard-Marangoni convection cells is solely dependent on the instant contact angle. The thermocapillary instabilities in the sessile droplet follow an evolution tendency consisting of three stages and two transition periods. As the contact angle decreases during evaporation, the dominant thermocapillary instabilities firstly transfer from Marangoni-capillary circulation to deformed Bénard-Marangoni cells at a growth rate of 7.9 per degree, and then to conventional Bénard-Marangoni cells. The decrease of deformed Bénard-Marangoni cells is almost at a constant rate of 1.5 per degree during the second stage. Details of flow fields and the corresponding interfacial temperature distributions are consistent with each other qualitatively and quantitatively. Two critical contact angles, 46∘ and 22∘, are found for the transitions of instabilities.

Modeling Unsteady B\xe9nard-Marangoni Instabilities in Drying Volatile Droplets on a Heated Substrate

We study unsteady internal flows in a sessile droplet of capillary size evaporating in constant contact line mode on a heated substrate. Three-dimensional simulations of internal flows in evaporating droplets of ethanol and silicone oil have been carried out. For describing the Marangoni flows we find it necessary to account for the diffusion of vapor in air, the thermal conduction in all three phases and thermal radiation. The equations have been solved numerically by finite element method using ANSYS Fluent. As a result of the simulations, the nonstationary behavior of Bénard-Marangoni (BM) instabilities is obtained. At the first stage, a flower structure of BM cells near the triple line emerge. For smaller contact angles, the cells grow in size and occupy the central region of the droplet surface. Being closely connected with recent experimental and theoretical studies, the results obtained help to analyze and resolve the associated issues.

Thermohydrodynamic processes of droplet evaporation and boiling on functional nanomaterials surfaces

We study the evaporation and boiling of water droplets on functional nanomaterial surfaces. Methods for manufacturing samples of nanocomposites, methods for studying contact angles, and evaporation dynamics are described. New experimental data on the behavior of contact angles and evaporation rates at various temperatures for samples of nanocomposites are presented in both the constant contact line mode and the constant contact angle mode. The features of evaporation for nanocomposites with inclusions of boron nitride and aluminum nitride nanoparticles inside graphene samples are noted.

Engineering Micropatterned Surfaces for Controlling the Evaporation Process of Sessile Droplets

Controlling the evaporation process of a droplet is of the utmost importance for a number of technologies. Also, along with the advances of microfabrication, micropatterned surfaces have emerged as an important technology platform to tune the wettability and other surface properties of various fundamental and applied applications. Among the geometrical parameters of these micropatterns, it is of great interest to investigate whether the arrangement of the patterns would affect the evaporation process of a sessile liquid droplet. To address this question, we fabricated four microhole arrays with different arrangements, quantified by the parameter of “eccentricity”. The results suggested that, compared to smooth substrates, the evaporation mode was not only affected by engineering the microhole arrays, but also by the eccentricity of these micropatterns. The values of contact angle hysteresis (CAH) were used to quantify and test this hypothesis. The CAH could partially explain the different evaporation modes observed on the microhole arrays with zero and non-zero values of eccentricity. That is, on microhole arrays with zero eccentricity, CAH of water droplets was comparatively low (less than 20 ° ). Consistently, during the evaporation, around 60% of the life span of the droplet was in the mixed evaporation mode. Increasing the eccentricity of the microhole arrays increases the values of CAH to above 20 ° . Unlike the increasing trend of CAH, the evaporation modes of sessile droplets on the microhole array with non-zero values of eccentricity were almost similar. Over 75% of the life span of droplets on these surfaces was in constant contact line (CCL) mode. Our findings play a significant role in any technology platform containing micropatterned surfaces, where controlling the evaporation mode is desirable.

Transition of Marangoni convection instability patterns during evaporation of sessile droplet at constant contact line mode

Due to continuous variation of contact angle of a sessile droplet during evaporation at constant contact line mode, the behaviors of Marangoni convection inside the droplet would inevitably vary with evaporation. In order to understand these behaviors, the Marangoni convection instability patterns and their transition phenomena in a sessile droplet during evaporation are investigated through a series of experiments. For easily volatile droplet of methanol the hydrothermal waves occur firstly near the three-phase contact line and then they are transformed to Bénard-Marangoni cells when the contact angle is lower than a critical value. The hydrothermal waves finally disappear with only Bénard-Marangoni cells remaining in the droplet when the contact angle is below the second threshold. For highly volatile droplet of R-113 the stationary longitudinal rolls appear firstly and then they are replaced by the Bénard-Marangoni cells. The basic linear flow in R-113 droplet caused by fast evaporation is a crucial reason to form the stationary longitudinal rolls in it. The critical conditions for the transitions between these kinds of the Marangoni convection instability patterns are determined and the underlying mechanisms of these transition phenomena are analyzed carefully.

Optical methods for droplets evaporation on graphene nanocomposite surfaces

We study the evaporation of water droplets on the surface of graphene nanocomposites of different compositions. Methods for manufacturing samples of nanocomposites, optical methods for studying contact angles and evaporation dynamics are described. New experimental data are presented on the behavior of contact angles and evaporation rates at various temperatures for samples of nanocomposites in both the constant contact line mode and the constant contact angle mode. The features of evaporation for nanocomposites with inclusions of boron nitride and aluminum nitride nanoparticles inside graphene samples are noted.

Hierarchical Rose Petal Surfaces Delay the Early-Stage Bacterial Biofilm Growth.

A variety of natural surfaces exhibit antibacterial properties; as a result, significant efforts in the past decade have been dedicated toward fabrication of biomimetic surfaces that can help control biofilm growth. Examples of such surfaces include rose petals, which possess hierarchical structures like the micropapillae measuring tens of microns and nanofolds that range in the size of 700 ± 100 nm. We duplicated the natural structures on rose petal surfaces via a simple UV-curable nanocasting technique and tested the efficacy of these artificial surfaces in preventing biofilm growth using clinically relevant bacteria strains. The rose petal-structured surfaces exhibited hydrophobicity (contact angle (CA) ≈ 130.8° ± 4.3°) and high CA hysteresis (∼91.0° ± 4.9°). Water droplets on rose petal replicas evaporated following the constant contact line mode, indicating the likely coexistence of both Cassie and Wenzel states (Cassie-Baxter impregnating the wetting state). Fluorescence microscopy and image analysis revealed the significantly lower attachment of Staphylococcus epidermidis (86.1 ± 6.2% less) and Pseudomonas aeruginosa (85.9 ± 3.2% less) on the rose petal-structured surfaces, compared with flat surfaces over a period of 2 h. An extensive biofilm matrix was observed in biofilms formed by both species on flat surfaces after prolonged growth (several days), but was less apparent on rose petal-biomimetic surfaces. In addition, the biomass of S. epidermidis (63.2 ± 9.4% less) and P. aeruginosa (76.0 ± 10.0% less) biofilms were significantly reduced on the rose petal-structured surfaces, in comparison to the flat surfaces. By comparing P. aeruginosa growth on representative unitary nanopillars, we demonstrated that hierarchical structures are more effective in delaying biofilm growth. The mechanisms are two-fold: (1) the nanofolds across the hemispherical micropapillae restrict initial attachment of bacterial cells and delay the direct contact of cells via cell alignment and (2) the hemispherical micropapillae arrays isolate bacterial clusters and inhibit the formation of a fibrous network. The hierarchical features on rose petal surfaces may be useful for developing strategies to control biofilm formation in medical and industrial contexts.

Marangoni instability induced by evaporation in well-defined non-spherical sessile droplet

Marangoni instability in well-defined non-spherical sessile droplet of 0.65cSt silicone oil evaporating at constant contact line mode is investigated through a series of experiments. Droplets of typical regular geometries (such as triangular, square, pentagonal and elliptical) and exotic shapes (such as kidney and positive-positive curvature) are tested. Cell patterns of Bénard-Marangoni instability are distributed along the azimuthal direction non-uniformly in the droplets. These cells always occur first at the region where the curvature of the three-phase contact line (kcl) is the largest and they do not fill the whole droplet at once. Different from that in spherical droplet, there is no such a steady state in the non-spherical droplets and these BM cells always drift from the lower kcl regions towards the higher ones. The influences of kcl, the substrate temperature and the contact angle on the patterns of the Bénard-Marangoni instability are analyzed. The critical contact angles for the onset of Marangoni instability are determined.

Study of Microdroplet Growth on Homogeneous and Patterned Surfaces Using Lattice Boltzmann Modeling

We present droplet growth dynamics on homogeneous and patterned surfaces (surface with hydrophilic and hydrophobic region) using two-dimensional thermal lattice Boltzmann method (LBM). In the first part, we performed 2D simulations on homogeneous hydrophobic surfaces. The result shows that the droplet grows at higher rate on a surface with higher wettability which is attributed to low conduction resistance and high solid–liquid contact area. In the later part, we performed simulations on patterned surface and observed that droplet preferentially nucleates on the hydrophilic region due to lower energy barrier and grows in constant contact line (CCL) mode because of contact line pinning at the interface of hydrophilic–hydrophobic region. As the contact angle reaches the maximum value of hydrophobic surface, contact line depins and droplet shows constant contact angle (CCA) growth mode. We also discuss the effect of characteristic width of hydrophilic region on growth of droplet. We show that contact angle of the droplet increases rapidly and reaches the contact angle of hydrophobic region on a surface with a lower width of the hydrophilic surface.

Influence of substrate temperature on Marangoni convection instabilities in a sessile droplet evaporating at constant contact line mode

Marangoni instabilities of a sessile droplet of 0.65 cSt silicone oil evaporating at constant contact line mode are experimentally investigated in a wide range of the substrate temperatures (Tw) from those lower than room temperature (Ta) to those higher than it. For Tw > Ta, the Bénard-Marangoni convection cells are observed and the cell patterns vary from the quasi-steady state to irregular oscillatory state with the droplet evaporation. For Tw < Ta, the irregular oscillatory Bénard-Marangoni cells are observed when Tw is higher than 10.6 °C. The cell size is larger while there are less cell numbers compared to that of Tw > Ta. When Tw is higher than 14.3 °C three kinds of convection mode, i.e. the travelling hydrothermal waves, the coexistence of hydrothermal waves and irregular oscillatory Bénard-Marangoni convection, and the irregular oscillatory Bénard-Marangoni convection, occur successively with droplet evaporating. The critical contact angles for the incipience of these Marangoni instabilities are determined.

Experimental study of evaporation of sessile droplets of the water-ethanol solution with different concentrations

The dynamics of the geometric parameters of droplets of a water-ethanol solution lying on a Teflon plate was investigated using high-speed microphotography. It was found that the evaporation of water droplets occurred with a constant contact line, and the evaporation of droplets of a water-ethanol solution was accompanied by its movement. At the initial stage of evaporation, the higher the concentration of ethanol in the drop was, the greater was the change in the contact spot diameter, corresponding to the change in the diameter of the spot of ethanol. Using the method of infrared thermography, three stages of temperature change of evaporating droplets were found. The first stage corresponds to the initial sharp decrease in temperature. The second stage is a process at a constant temperature. And finally, the third stage is the stage of its smooth growth to the ambient air temperature. A similar pattern with some features was observed for droplets with different concentrations of ethanol. At the initial stage of evaporation, the droplet temperature changed like the temperature of ethanol droplet, and then its behavior was similar to the temperature change of water droplets. The higher the ethanol concentration in the droplet was, the larger the change in temperature was, similar to the change in the temperature of the ethanol droplet.

Evaporation characteristics of fuel sessile droplets with nanoparticles

ABSTRACTThe nano-fuel are prepared by dispersing CNT, CeO2, and Co3O4 nanoparticles in n-tetradecane (C14). This experimental study investigates the effect of nanoparticles mass fraction as well as the variation of nanoparticles sizes on the viscosity and evaporation characteristics. It is revealed that the viscosity increases with the increase in mass fraction and the decrease in particle size. In addition, the presence of nanoparticles leads to a reduction of the evaporation rate during constant contact line phase. However, the droplet evaporation rate is augmented remarkably in constant contact angle and hybrid phase. As a result, the whole evaporation time of nano-fuel droplets is shorter than base fuel.

3D lattice Boltzmann investigation of nucleation sites and dropwise-to-filmwise transition in the presence of a non-condensable gas on a biomimetic surface

Condensation in the presence of non-condensable gas on a biomimetic pillared subcooled surface with hybrid wettability (with hydrophilic top and hydrophobic side and bottom) is investigated using a newly developed 3D multi-component multiphase lattice Boltzmann model. Three preferred nucleation sites with different surface wettability contrasts are found: (i) at the corner of side and bottom hydrophobic surface; (ii) at the center of the bottom hydrophobic surface, and (iii) on the top hydrophilic surface of the pillar. Influencing factors, such as pillar geometrical parameters, subcooling degree and non-condensable gas concentration, on dropwise-to-filmwise condensation transition are examined. For dropwise condensation on a top hydrophilic pillar surface, the droplet undergoes a two-stage growth pattern from “changing contact line” to “constant contact line”. Non-condensable gas is found to aggregate near the condensing interface in the vapor phase and at corners of pillar side surface and bottom surface. Increasing pillar width or pillar height (with other geometric parameters remained unchanged), as well as decreasing degree of wall subcooling or non-condensable gas concentration, can delay transition from dropwise to filmwise condensation.

Variable height modification of TA worm drive

The related research shows that a constant contact line, which is negative for the normal operation of the worm gearing, exists on the middle of the unmodified and the constant height modified TA worm wheel surface. To overcome this drawback, a variable height modification method is proposed for the TA worm drive. In line with this modification method, the height modification parameter is variable during the whole processing cycle. Accordingly, the obtained gearing can be named as the variable height modified TA worm drive. The mathematical model for the meshing analysis of this novel worm drive is established according to its generation mechanism and the mesh theory of gearing. The reason why the variable height modification can remove the constant contact line on the worm gear tooth surface is analyzed in detail. In addition, the classification criterion of the transmission type is also derived. The computing methods of the key points on the contact zone boundary and the instantaneous meshing line are elaborated. On the basis of the above theoretical analysis, the meshing characteristics of this novel worm gearing are well investigated. The results manifest that the above theoretical analysis is valid, and the obtained gearing has favorable meshing properties. Furthermore, it is pointed out that the linear variable height modification with larger quantity can be recommended as the ideal strategy in practice.

Influence of fluorinated self-assembled monolayer on wetting dynamics during evaporation of refrigerant–oil mixture on metal surface

Reducing wettability of a metal surface is a promising method for enhancing boiling heat transfer of refrigerant–oil mixture on the metal. As fluorinated self-assembled monolayer (F-SAM) coating is effective for wettability reduction, its influence on wetting dynamics including meniscus shape, contact angle, contact line velocity and rising liquid height during evaporation of refrigerant–oil mixture on metal surface were experimentally investigated. The refrigerant–oil mixture was prepared by R141b and NM56, the oil mass fraction ranged from 0 to 10 wt%, and the surface roughness ranged from 0.028 to 1.166 µm. The results show that during evaporation of refrigerant–oil mixture, the presence of F-SAM changes the evaporation mode to be constant contact line velocity followed by both constant contact angle and contact line velocity, while decreases the rising liquid height. The results suggest that larger surface roughness and higher oil mass fraction are preferred when using F-SAM to reduce surface wettability.