Display omitted
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.
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 The surface was immersed into or withdrawn from an aqueous glycerol-water solution by varying the capillary number and the lubricant viscosity.
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 θD3~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.
Water management in the flow field as well as the flooding process in the gas diffusion and catalyst layers enormously influence proton exchange membrane fuel cells (PEMFCs) performance and ...reliability. Researchers have developed many different designs for flow channels that can be used to distribute fuel or oxidant in PEMFCs (proton exchange membrane fuel cells). Among these designs, novel biomimetic designs have captured special attentions from researchers due to their capability of distributing fluids effectively. This study presents an investigation of the liquid water transport within a porous layer and a symmetrical biomimetic flow field based on Murray's law. The volume of fluid (VOF) method is employed, and the dynamic contact angle (DCA) effects are also considered for better prediction of water distribution. The water transport process and water distribution inside the porous layer and flow field are obtained from the simulation results. Recommendations are given for this type of flow field design based on the behaviors of liquid water in the porous layer and flow field.
Display omitted
•Murray's Law is used for designing a biomimetic flow field.•Water transport behaviors are investigated using the Volume of Fluid method.•Dynamic contact angle model is implemented in the simulation.•The sharp 90° degree bends cause the ununiform in the water and pressure distributions.
•A new solution for spontaneous imbibition with dynamic contact angle and gravity.•Dynamic contact angle causes a nonlinear curve of imbibition rate in a log-log plot.•A core-scale spontaneous ...imbibition model with dynamic contact angle and gravity.•Initial imbibition rate is dominated by maximum pore radius and fractal dimension.
Liquid imbibition with dynamic contact angle is a ubiquitous phenomenon of fluid flow in porous media, but its analytical solution is challenging to derive. In this work, the analytical solution of liquid imbibition in an inclined capillary tube with velocity-dependent contact angle and gravity is first derived. The time required for the liquid-gas interface to reach a certain distance by spontaneous imbibition with static contact angle and dynamic contact angle are provided. Assuming that the porous medium consists of a bundle of tortuous capillary tubes with fractal distribution of pore size, a mathematical model of liquid imbibition in core-scale porous media with dynamic contact angle and gravity is developed. The studies show that the dynamic contact angle can significantly reduce the imbibition velocity at the initial stage of imbibition, especially in large pores, which forms a nonlinear correlation between imbibition velocity and imbibition time in a log-log plot.
Display omitted
•Spreading dynamic of a deposited drop on a solid substrate is studied.•Viscous timescale mainly governs the spreading at a certain Bond number and static contact angle.•Droplet ...spreading master curves to capture the dynamic shape of the droplet are developed.•A correlation to predict the equilibrium shape of droplet is developed.
There exists a generalized solution for the spontaneous spreading dynamics of droplets taking into account the influence of interfacial tension and gravity.
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.
We show that the spreading process is governed by three important parameters: the Bo number, viscous timescale τviscous, and static advancing contact angle, θs. More specifically, there exists a master spreading curve for a specific Bo and θs by scaling the spreading time with the τviscous. Moreover, we developed a correlation for prediction of the equilibrium shape of the droplets as a function of both Bo and θs. 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.
•Developed a multiphysics-multiphase model capable of simulating a droplet impinging a surface.•Model comprises fluid dynamics, heat transfer, contact angle, solidification, adhesion & other.•Model ...predictions were carefully validated against our new nanocomposite coating.•Work highlights the important effect of using the correct wettability parameters on model accuracy.•Inclusive effort that provides an insight into rebound/adhesion of a droplet impinging a surface.
This multiphysics-multiphase numerical study is concerned with the fundamental understanding of the mechanisms associated with the impingement of a supercooled water droplet on superhydrophobic and icephobic surfaces. Specifically, the current finite volume simulations, which incorporate the volume of fluid method (VOF), account for a number of highly coupled physical processes concurrently. These include fluid dynamics, heat transfer, dynamic contact angle, and solidification under the effect of momentum, surface tension, wall adhesion, and gravitational forces. It also overcomes a number of shortcomings that currently exist in the literature that lack the accurate description of surface wettability. The predictions of the model were compared with the findings of an extensive experimental test program involving newly developed superhydrophobic and icephobic coatings. The model predictions were found to be in excellent agreement with the experimental findings. Specifically, our results reveal that as the repellency features of the surface are reduced, the droplet experiences a longer spreading phase, larger spreading diameter, and increased loss in kinetic energy due to the greater wall adhesion and freezing of the interfacial contact surface. These effects would ultimately lead to droplet pinning instead of rebounding. The findings of this research are crucial to anti-icing applications.
Display omitted
•A function of the critical airflow velocity that droplet onset is established.•The process of droplet motion is divided into four stages.•The wetting length of droplet motion is analyzed in each ...stage.•The factors that affect the velocity of the center of droplet gravity are explored.•The change of dynamic contact angle of droplet on different surfaces is analyzed.
The movement and deformation of droplet is widely applied in many fields. In this study, to investigate the dynamics of droplets moving on surfaces with different wettability driven by airflow, a series of experiments are conducted by using a high-speed camera, under conditions of airflow speeds increased from 0m/s to 22.2m/s with the same pattern per case, droplet volume varied from 20 to 80 μL, and different wettability surfaces (aluminum, PMMA, titanium, PTFE, PAL). Based on the observation of experiments, the change of the morphology of the droplet motion is divided into 4 stages. Stage I: Droplet oscillates at the original position and its wetting line does not move. Stage II: The droplet still oscillates and its wetting line slightly moves on the surface. Stage III: Droplet deforms dramatically and accelerates forward, and its wetting line elongates sharply. Stage IV: Several sub-droplets are split from the mother droplet. The droplet separation usually occurs after its wetting length reaches the extremum value. Specifically, the droplet separation is not observed on the PTFE and PAL surface, and the four stages of droplet motion reduce to three in these cases. According to the observation, droplet shapes during their movement are classified into four categories including compact, tail formation, film formation, and droplet separation. The relationship between the critical airflow speed at which the wetting line begins to move and the droplet diameter is studied, corresponding to different surfaces. To provide an insight into the phenomenon of dynamics and deformation of airflow-driven sessile droplet spreading, quantitative analysis of the characteristic parameters of the droplet including the wetting length, the dynamic contact angle, and the velocity of the center of droplet gravity are conducted. It is shown that the wetting length ratio of droplets moving on different surfaces varied in the range from 0 to 3.63. The maximum value of the difference between cosines of the upstream contact angle and downstream contact angle usually approaches the cosines of the receding contact angle and the advancing contact angle at the end of Stage II. This study provides experimental reference data for the study of the droplet motion driven by airflow.
Dynamics of Droplet Detachment in REW.
Display omitted
Droplet actuation by electrowetting (EW) has drawn significant interest due to the potential applications in micro- and nano-fluidics, and the ...droplet departure is crucial for separation of the drop phase from the solid surface. However, the operating condition for droplet detachment, induced by the traditional electrowetting is quite strict, making it inconvenient for practical application. Recently, we considered the reversed electrowetting (REW) phenomenon, where a non-conductive droplet, settled on an adhesive surface, is dewetted continuously by applying the potential different between the substrate and surrounding fluid (Wang et al. 2020). Detachment of the oil drop is induced naturally and the detaching process is controllable. We have investigated the physical process of REW in the previous experiments. However, the dynamics of droplet detachment and the underlying mechanism are not well explained by the macroscopic experimental approach. For complementation, we build a numerical scheme in this study and examine the transient dynamics of droplet motion in the REW. The interface of the liquid-liquid system is captured by a phase-field method, and a correlation for the dynamic contact angle is incorporated in the numerical model. The simulated results are validated with the experimental cases. The process of droplet detachment and the critical condition are investigated by analyzing the energy transition during the droplet motion.
Display omitted
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.
•Droplet impact under different roughness and vibration conditions are studied.•Higher surface roughness inhibits droplet spreading.•Vibration results in a more pronounced variation in the dynamic ...contact angle.•Correlation was proposed for droplet spreading with an error of ±9.5%.
Droplet impingement is pivotal in several fields, such as agricultural spraying, ink printing and firefighting. Although droplet impact studies have received much attention, the combined effects of roughness and vibration on droplet dynamics still need to be explored. In this study, an in-depth analysis is carried out to address this issue, and high-speed photography is utilised to explore the droplet impact process under different roughness and vibration conditions. The results show that higher surface roughness dissipates the kinetic energy of droplet spreading, thus inhibiting droplet spreading. Conversely, vibration imparts additional inertial effects to the droplet, leading to sustained oscillations once the droplet stabilises, which in turn results in a more pronounced variation in the droplet's dynamic contact angle. Therefore, in this paper, the vibrational Weber number (Wev) is employed to characterise the interaction between the vibration-induced extra inertia force and the droplet's surface tension. Moreover, as the Wev increases, the relative velocity of the droplet impact also rises, further intensifying the droplet's spreading behaviour. At a Wev of 8.72 and a surface roughness of 0.4 μm, droplet spreading is enhanced by 33%. While at a surface roughness of 6.3 μm, droplet spreading was suppressed by 32% relative to a surface roughness of 0.4 μm. Ultimately, an empirical model was proposed to predict the droplets' maximum dimensionless spreading coefficient (βmax) under vibration conditions, with an error of ±9.5% in the model prediction. This study delves into the dynamic characteristics of droplets under the combined influence of surface roughness and vibration, offering crucial theoretical foundations and profound insights for engineering applications in the field.
Abstract Cleats are the main channels for fluid transport in coal reservoirs. However, the microscale flow characteristics of both gas and water phases in primary cleats have not been fully studied ...as yet. Accordingly, the local morphological features of the cleat were determined using image processing technology and a transparent cleat structure model was constructed by microfluidic lithography using the multiphase fluid visualization test system. Besides, the effect of microchannel tortuosity characteristics on two‐phase flow was analyzed in this study. The results are as follows: (1) The local width of the original cleat structure of coal was strongly nonhomogeneous. The cleats showed contraction and expansion in the horizontal direction and undulating characteristics in the vertical direction. (2) The transient flow velocity fluctuated due to the structural characteristics of the primary cleat. The water‐driven gas interface showed concave and convex instability during flow, whereas the gas‐driven water interface presented a relatively stable concave surface. (3) The meniscus advanced in a symmetrical pattern in the flat channel, and the flow stagnated due to the influence of undulation points in a partially curved channel. The flow would continue only when the meniscus surface bypassed the stagnation point and reached a new equilibrium position. (4) Enhanced shearing at the gas–liquid interface increased the gas‐injection pressure, which in turn increased residual liquids in wall grooves and liquid films on the wall surface.
Highlights Geometric characterization and reconstruction of a three‐dimensional transparent cleat model were performed. Gas–liquid repulsion characteristics were determined in a visualized microcleat model. Transient flow velocity hysteresis is evident in tortuous channels. Dynamic variation of the contact angle shows diversity. Pressure changes the groove liquid content and liquid film thickness.
