Droplet Dynamic Behavior Research Articles

Effects of droplet dynamical behavior in flow channel of proton exchange membrane fuel cell under vibration conditions on cell performance

Proton exchange membrane fuel cells are subjected to severe vibration in aerospace applications. Understanding the droplet transport characteristics in the flow channelunder vibration conditions is essential for improving cell performance. However, the underlying patterns and mechanisms of the vibration effects on cell performance and its internal water transport remain unclear, and there is a lack of improvement methods for drainage under vibration conditions. This study experimentally investigates the effect of vibration on cell performance. Experimental results indicate that vibration predominantly affects cell performance through its modulation of water transport. Therefore, the droplet dynamic behavior in the flow channel is further studied using the volume of fluid method. The results indicate that vibrations can enhance the average performance of the fuel cells, with the largest increase of 8.30 %. This is attributed to vibrations promoting the detachment of droplets from the gas diffusion layer surface and reducing the water coverage ratio on said surface. Under conditions of low gas velocity, high surface hydrophobicity, and large droplet size, droplets exhibited oscillatory and jumping behavior in the flow channel due to vibration, resulting in the largest 53.5 % increase in peak water coverage ratio on the gas diffusion layer surface compared to no-vibration conditions. Furthermore, enhancing the hydrophobicity of the gas diffusion layer and the hydrophilicity of the channel walls promotes droplet detachment and discharge compared to no-vibration, as well as contributing to the improvement of vibration effects on the drainage process.

Development of self‐cleaning and antibacterial properties on cotton fabric using silver nanoparticles and PFOTS

AbstractThe demand for superhydrophobic fabric with self‐cleaning and antibacterial features as potential practical applications has been rising steadily with the passage of time. In this research work, the wet chemical method is used to manufacture cotton with a superhydrophobic coating. Thus, Nanoparticles of silver were produced by using an in situ method. Afterward, the fabric is treated with a solution of perfluorooctyltriethoxysilane to make it superhydrophobic. The cotton fabric performs remarkably with exceptional hydrophobicity as it had a contact angle of 157 ± 1° and less than 10° in sliding angle. Water droplet's dynamic behavior is also studied so that the superhydrophobic fabric development can be proven. The fabric shows excellent chemical durability by performing chemical tests. The antimicrobial and self‐cleaning properties are greatly enhanced by the superhydrophobic coating on cotton. Particle density on the superhydrophobic cotton before and after self‐cleaning was found 5.1 and 0.4 mg/cm2. In addition, the treated cotton fabric also exhibited superior antibacterial properties against Escherichia coli bacteria. Therefore, the developed multifunctional cotton fabric can be used in real‐life applications.Highlights Cotton fabric enables to repelling of dirt, oil, and water‐based stains more effectively. Silver nanoparticles possess potent antibacterial properties. Incorporation of PFOTS controls the release of silver into the environment. Versatile applications from healthcare and hospitality to outdoor and athletic apparel.

Regulation of droplet dynamic behavior after droplet impact on superhydrophobic surfaces

Controlling the impact behavior of a droplet on a superhydrophobic surface is a significant challenge for many applications. Instead of texturing the surface, this study investigated the effect of adding surfactants on the morphology of the droplet after it impacts superhydrophobic surfaces. The results showed that adding low concentrations of sodium dodecyl sulfate (SDS) surfactant prevents fragmentation and splashing. High concentrations of SDS can inhibit rebound and allow droplets to be easily deposited on surfaces after impact. When the droplet size was 3.51 mm and the SDS concentration was above the critical micelle concentration, the contact time of the solution increased by a maximum of 1.30 times compared to that of pure water. In a high-concentration SDS solution, the surfactant molecules can quickly migrate to the newly generated interface during the spreading stage, reducing the interfacial tension and causing wetting changes. The results of this study are expected to contribute toward establishing a simple method to inhibit splashing and increasing contact time on superhydrophobic surfaces.

Experimental Study on the Droplet Transport and Dynamic Behavior in Flow Channel with Microprotrusions of PEMFC

Experimental Study on the Droplet Transport and Dynamic Behavior in Flow Channel with Microprotrusions of PEMFC

Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface

To provide further insight into humid air condensation on hybrid surfaces, an experiment was conducted to visually investigate the condensation process on wettability-interval grooved surfaces, which had hydrophobic ridges and hydrophilic grooves. The droplet dynamic behavior and heat transfer performance of condensation on a wettability-interval grooved surface were explored and compared with four other functional surfaces, including the plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, and hydrophobic grooved surface. The presence of hydrophobic ridges perpendicular to the groove direction and hydrophilic grooves allowed for the exclusion and easy spreading of droplets, respectively. Compared with the other four functional surfaces, the coupling phenomena during condensation, i.e., the spontaneous suction and directional drainage via hydrophilic grooves, were only found on the wettability-interval grooved surface. These could not only remove condensate quickly but also suppress the formation of the flooded liquid film, which was beneficial to the enhancement of heat transfer performance. It was proven by the experimental results that at subcooling 12 K, the condensation heat flux on the wettability-interval grooved surface reached 1280 W/m2, which was 1.25 times that of the plain hydrophobic surface (1030 W/m2), and 15% higher than that of the hydrophobic grooved surface (1110 W/m2). This indicated that the wettability-interval microgrooves could effectively enhance humid air condensation heat transfer performance.

The dynamic behavior of a self-propelled droplet on a conical fiber: A lattice Boltzmann study

In this paper, the dynamic behavior of a self-propelled droplet along a conical fiber is simulated by using an improved lattice Boltzmann color-gradient method. This method is developed on the basis of our recently developed density ratio model [Zhang et al., Int. Commun. Heat Mass Transfer, 137, 106284 (2022).], but a wetting boundary condition is added to account for the moving contact line on an arbitrary solid surface. First, this method is validated against the analytical droplet shapes and contact angles for droplets surrounded by matrix fluids of different densities on flat and spherical surfaces, and the spontaneous transport of a droplet on a conical fiber. This method is then adopted to systematically study the effects of the Bond number (Bo), surface wettability (θ), cone hemi-angle (α), and droplet volume on the droplet dynamic behavior. In each case, the results show that the droplet climbing velocity first increases and then decreases, and a velocity fluctuation is observed, which is due to that the apparent receding and advancing contact angles do not simultaneously reach the equilibrium contact angle. As droplet volume increases, the equilibrium droplet height monotonically increases. As Bo or θ increases, the droplet climbing height and the wetting area both decrease. We also found that the equilibrium climbing height first increases and then decreases with α, and its maximum is reached around α=2.5°.

Molecular Dynamics Simulation of Droplet Impact on a Hydrophobic 3D Elastic Surface.

The phenomenon of droplets impacting elastic surfaces is common in nature and in many engineering applications. It has been shown that droplet impact on an elastic surface drastically reduces droplet contact time and hinders droplet spreading. However, most of the current studies are based on experiments, and the analysis of the influence mechanism of the elastic substrate on the dynamic behavior of droplets is not complete. In addition, the simulations of droplet impact on elastic substrates are mainly focused on 2D elastic films or vibrating rigid substrates, ignoring the effect of 3D elastic substrate deformation on the droplet dynamic behavior. Therefore, in this paper, we propose to model the droplet impact on a 3D hydrophobic elastic substrate using the molecular dynamics method. We find that droplet pancake rebound can substantially reduce the droplet contact time. Moreover, we record the conditions required for the pancake rebound of the droplet. Furthermore, we investigated the effects of the elastic modulus of the substrate and the initial velocity of the droplet on the droplet contact time, contact area, and spreading factor. This study further elucidates the influence mechanism of the elastic substrate on the dynamic behavior of the droplet and provides theoretical guidance for regulating the dynamic behavior of the droplet in related fields.

Transient study of droplet oscillation characteristics driven by an electric field

Electrowetting technology, a microfluidic technology, has attracted more and more attention in recent years and has broad prospects in terms of microdroplet drive. In this paper, the dynamic contact angle theory is used to develop a numerical model to predict the droplet dynamic contact behavior and internal flow field under electrowetting. In particular, based on the established computational model of droplet force balance, the dynamic process of a droplet under electrowetting is analyzed, including the perspective of pressure variation and force balance inside the droplet. The results show that when the alternating current frequency increases from 50 Hz to 500 Hz, the amplitude of the oscillation waveform after droplet stabilization is 0.036 mm, 0.016 mm, 0.013 mm and 0.002 mm, while the relevant droplet oscillation period T is 11 ms, 4 ms, 2 ms and 1 ms, respectively. It is also found that the initial phase angle does not affect the droplet oscillation amplitude. In addition, the pressure on the droplet surface under alternating current electrowetting increases rapidly to the maximum value with resonant waveform oscillation, and the droplet will present different resonance modes under voltage stimulation. The higher the resonance mode is, the smaller the droplet oscillation amplitude is and the streamline at the interface will present an eddy current, in which the number of vortices matches the resonance mode. A high resonance mode corresponds to a small droplet amplitude, while there are more vortices with a smaller size.

A novel cathode flow field for PEMFC and its performance analysis

A good flow field design is important to the proton exchange membrane fuel cell (PEMFC) performance, especially under a high current density region, which is dominated by concentration polarization. Motivated by the variable cross-section channel idea, in this study, a novel flow field containing a converging-diverging (C-D) pattern is proposed. A three-dimensional multiphase model is established to study its performance. The numerical results show that it outperforms the conventional straight channel and only depth-variant channel. In the novel flow field the enhanced under land cross flow and higher effective mass transfer coefficient both improve the reactant transport. The effects of operating conditions, like stoichiometric ratio and operating pressure, on cell output performance are studied. It is found that a higher promotion rate can be obtained by increasing the stoichiometric ratio, but increasing the operating pressure has little effect. The droplet dynamic behavior in the C-D channel and straight channel are studied, and the results prove the better drainage capability of the novel flow field.

Performance transition in droplet-based electricity generator with optimized top electrode arrangements

Harvesting electrical energy from water droplet is a sustainable and promising way to meet challenges in energy scarcity. The water kinetic energy has been successfully harvested into electricity energy by droplet-based electricity generator (DEG). By virtue of its novel transistor-like design with a top electrode, the DEG achieves a high instantaneous power density basing on bulk effect instead of interfacial effect. Despite this, how top electrode arrangement influences the electricity generation as a basic question is still obscure. Here, we study the impact of top electrode arrangement on the output performance of DEG. A transition between open system and closed system is found in the generator when the top electrode arrangement changes at vertical and horizontal direction. Such transition is further analyzed from the working mechanism, droplet dynamic behavior and output performance. Finally, an optimal top electrode arrangement is acquired, which will provide guidance in DEG design. We envision that such optimal top electrode arrangement of DEG will open new pathway for efficiently harvesting water droplet kinetic energy.

Three-dimensional study of double droplets impact on a wettability-patterned surface

In this paper, a three-dimensional multi-relaxation-time pseudopotential lattice Boltzmann model is adopted to study the double droplets impact on a wettability-patterned surface. The effects of several factors like the wettability difference, the Weber number and the droplet spacing on the droplet dynamic behavior are studied in detail. The numerical results show that the unbalanced Young’s force caused by the wetting difference will induce the droplets to rebound or migrate laterally toward the region with high-wettability. In addition, there exist three evolutionary phases for the double droplets impact on the wettability-patterned surface: the asymmetric spreading phase, asymmetric retracting phase, and wetting equilibrium phase. Further, when the Weber number is relatively smaller, a secondary spreading behavior could be observed which in turn causes an increase in the contact time. Finally, as far as the influence of the droplet spacing is concerned, we find that the flow patterns for different droplet spacings are largely different, and the presence of a air cavity could reduce the contact time.

Modelling study on freezing process of water droplet on inclined cold plate surface with droplet dynamic behavior considered

Droplet freezing on inclined surfaces exists widely in engineering fields. To accurately predict and control the freezing process of a sessile water droplet on inclined surface, a theoretical model based on the heat-enthalpy method is presented in this study, with two types of dynamic behavior considered, deformation and spreading. After the validation of model by droplet profiles and freezing duration from experiments, the freezing characteristics are analyzed, including contact area, frozen height and vertex offset, etc. As found, the effect of inclined angle on less than 10.34 µL water droplet is greater than that on larger than 10.34 µL droplet, due to the mutual relation between surface tension and gravity effect. When the inclined angle of surface changes from 0° to 40°, the contact area keeps at 11.61 mm2 for 10 µL water droplet, and increases by 5.64% from 23.04 to 24.34 mm2 for 25 µL water droplet. The initial heights of 10 and 25 µL water droplets decrease by 0.85% from 1.18 mm to 1.17 mm and by 1.91% from 1.51 mm to 1.49 mm, respectively. That means it is easier frozen for the same water droplet on bigger inclined angle surface. This study is beneficial for the optimization of anti-frosting and defrosting technologies.

On the droplet impact dynamics of nonionic surfactant solutions on non-wettable coatings

HypothesisThe impact dynamics of droplets containing nonionic surfactants are dependent on their molecular weight and concentration. This study investigates the role of these key parameters on the droplet dynamic behavior of nonionic surfactant solutions on non-wettable alkyl ketene dimer surfaces. ExperimentsAqueous solutions of 3 nonionic surfactants, MEGA-10, Brij C10 and Brij O20, with molecular weights of ∼ 350, 683 and 1150 g/mol, were prepared at various concentrations. The droplet impact dynamics of these solutions were observed using a high-speed camera. FindingsThe results show that neither the concentration nor the molecular weight can noticeably change the spreading of the surfactant-laden droplets. A strong correlation was established between droplets’ maximum spreading factor (βmax) and Weber number (βmax = 0.76We0.29). In addition, with a slight modification, we adopted two existing analytical models to predict βmax. While the model of Pasandideh-Fard et al. overpredicts βmax, especially at We < 40, the statistical analyses showed the reasonable accuracy of the model of Ukiwe and Kwok. Furthermore, as the molecular weight decreases, the surfactants more noticeably weaken the vertical oscillation of the droplets in the retraction phase. Surfactant solutions can act similar to dampers and limit the complete/partial rebound of the droplets with no effect on their splashing behavior.

Simulation investigation of the spontaneous motion behaviors of underwater oil droplets on a conical surface.

A conical surface can realize the spontaneous transportation of micro-sized oil droplets in an aqueous environment without energy input, exhibiting great potential for applications in microfluidics, chemical micro-reactors, water remediation, etc. However, the precise manipulation of an oil droplet on a cone is still very challenging because the dynamic behavior of a droplet on a cone is not fully understood. Herein, the dynamic behavior of oil droplets on a cone is quantitively studied via numerical simulations, and the effects of wettability, apex angle, and droplet size on the droplet's dynamic behavior are systematically analyzed. The results show that the moving velocity and transport distance of the droplet on the cone are highly related to the droplet shape on the cone. It was found that a clamshell-shaped droplet moves faster than a barrel-shaped droplet. Besides, the clamshell-shaped droplet with a larger size, on the cone with a smaller apex angle and smaller contact angle tends to obtain a faster moving speed and a longer transportation distance. The droplet shape adopted on the cone was determined by the cone wettability and the size of the droplet relative to the local curvature of the cone. It was found that the oil droplet tends to form a barrel shape on the cone with a highly oleophilic and small apex angle, and tends to form a clamshell shape on cones with a highly oleophobic and large apex angle. In addition, the droplet might transit from a barrel shape to a clamshell shape when it moves from the cone tip to the cone base, and the trigger time of the transit is negatively correlated with the contact angle and apex angle of the cone. This work provides a microscale understanding of the dynamic behavior of an underwater oil droplet on a cone, and also offers theoretical guidance for manipulating the behavior of a droplet on a cone and for the rational design of cone surfaces for spontaneous droplet transport and droplet collection.

Experimental Study on the Droplet Transport and Dynamic Behavior in Pem Fuel Cell Flow Channel with Micro-Protrusions

Experimental Study on the Droplet Transport and Dynamic Behavior in Pem Fuel Cell Flow Channel with Micro-Protrusions

Lattice Boltzmann simulation of droplet impacting on the superhydrophobic surface with a suspended octagonal prism

The contact time of a droplet impacting on a superhydrophobic substrate is demonstrated to be reduced via adding a macro-wire or a rectangular ridge, which is achieved by triggering the asymmetric feature during the spreading/retraction processes. Here, we use the lattice Boltzmann method to study the droplet impacting dynamics on a superhydrophobic surface with a suspended octagonal prism. We reveal that the asymmetric retraction of the impacting droplet strongly depends on the combined effect of Weber number and the suspended height. The falling droplet is split into two sub-droplets and shows growing asymmetry with increasing Weber number on the surface, during which the contact time reduces because the asymmetry results in an uncompleted retraction process only perpendicular to the prism of the sub-droplets. The study also shows that the prism suspension height remarkably affects the droplet dynamic behavior. A non-bouncing region is found at h/D0 (h is the prism height, and D0 is the initial diameter of droplet) in the range of 0.3–0.8 where the split droplets merge again and wrap the prism so that the attenuation in upward and transverse velocity is unable to tear up the liquid bridge beneath the prism. The prism height larger than ∼0.75 is shown to have limited influence on the contact time variation. Our study shows that at high We = 103.3, the ratio of prism height to the droplet diameter h/D0 is 0.2 or larger than 0.75 and the contact time significantly reduces by ∼59% compared to the flat surface.

Experiment on the quasi-superhydrophobic levitation state of droplets with low surface tension

<p indent=0mm>Droplet wettability and dynamic behavior of working fluids with low surface energy are widely used in various industrial and advanced technology fields, such as refrigerant spray cooling and microfluidic chip. Their wettability and dynamic behavior play a critical role in improving the efficiency of industrial processes. Some scholars have realized the wettability control of droplets of low surface energy fluids by changing the characteristics of micro-nano structures on solid surfaces. In addition, many external incentive ways, such as electrowetting, magnetic field, thermal stimulation, and supersonic vibration, have been proposed to control the droplet wetting state and motion behavior. However, such methods have limitations and no significant effect on low surface energy working fluids. Inspired by the above methods and the Leidenfrost phenomenon, this study made use of the action of the microporous airflow, which can regulate the wettability and motion behavior of low surface tension droplets without adding additional components to the droplets and without special requirements for the droplets themselves. A visual experimental system for wetting and dynamic behavior of droplets regulated by the microporous airflow was set up, and the experimental section of the regular array micropore was machined by laser. The momentum force generated by the microporous flow was used to balance droplet gravity, which overcomes the wetting behavior of the low surface tension droplets FC3283 and FC40 and the realizes the quasi-superhydrophobic levitation state of droplets with low surface tension. This study aimed to explore the regulatory mechanism of the microporous flow additional momentum force on the wettability and dynamic behavior of droplets with low surface energy. Experimental results showed that the motion behavior of droplets in the quasi-superhydrophobic levitation state could be divided into four stages (i.e., hovering, bobbing, bouncing, and detaching) with the decrease in droplet radius. The motion behavior of droplets was related to the dynamic balance of gravity, microporous flow momentum force, inertia force, and surface tension of droplets. In the bouncing stage, mutual conversion of energy was observed in the process of droplet falling and rising. A positive correlation was found between gravitational potential energy and surface energy. This kind of energy conversion made droplets oscillate periodically at a stable frequency. In addition, the deformation degree and amplitude of droplets with two kinds of low surface energy working fluids during the bouncing stage were compared, and the effect of viscosity on the droplet energy conversion process was analyzed. For wettability research, analogous to Young’s equation, combined with the force analysis of droplets at the inflection point during maximum deformation, the concept of pseudo contact angle was proposed to describe the wetting behavior of droplets. The pseudo contact angle of droplets in different oscillation stages was analyzed. In the hovering stage, the pseudo contact angle of droplets reached 153.6°. In the bouncing stage, as the droplet radius and falling velocity increased, the inertia of droplets increased and the surface tension decreased. Therefore, with the increase in Weber number, the pseudo contact angle of droplets decreased gradually. This method can supplement the external field force droplet manipulation method, and it provides a new method for potential applications in low surface tension droplet manipulation, lossless rapid directional transport, and microfluidic system design.

Lattice Boltzmann simulation of three-phase flows with moving contact lines on curved surfaces.

A numerical method for simulating three-phase flows with moving contact lines on arbitrarily complex surfaces is developed in the framework of the lattice Boltzmann method. In this method, the immiscible three-phase flow is modeled through a multiple-relaxation-time color-gradient model, which not only allows for a full range of interfacial tensions but also produces stable outcomes for a wide range of viscosity ratios. A characteristic line model is introduced to implement the wetting boundary condition, which is not only easy to implement but is also able to handle arbitrarily complex boundaries with prescribed contact angles. The developed method is first validated by the simulation of a Janus droplet resting on a flat surface, a perfect Janus droplet deposited on a cylinder, and the capillary intrusion of ternary fluids for various viscosity ratios. It is then used to study a compound droplet subject to a uniform incoming flow passing through a multipillar structure, where three different values of surface wettability are considered. The simulated results show that the surface wettability has significant impact on the droplet dynamic behavior and final fluid distribution.

The transient flow mechanism and intermittent film thickness evolution regulation of droplet mode for horizontal tube falling film

The behavior of falling film over horizontal has significant impacts on the efficiency of heat and mass transfer for the horizontal tube evaporators and absorbers especially for intermittent droplet model at low spray density, which has lower heat and mass transfer resistance. Thus it is critical to illuminate transient and intermittent flow mechanism of droplet dynamic behavior and film thickness evolution regulation over horizontal tube falling film at low spray density. A two dimension multi-phase flow mode for superhydrophilic surface was established to capture the microscopic flow mechanism. The results indicated that droplet generation and growth stages accounted for more than half of the droplet life cycle and an obvious droplet retraction and pulsation were first observed during droplet generation due to surface tension and local additional pressure. Further, the whole life cycle transient and intermittent film thickness map of droplet mode was depicted. And found that the liquid film thickness transient evolution regulation of droplet mode could be split into two zones: impact zone and stable zone. For impact zone, the film thickness was rapidly decreased with the spreading of the liquid film from the millimeter level to 200 μm within 0.2 s. While for stable zone, film thickness almost maintained stable over a long period time until the next droplet impacted. Because of the intermittent of droplet generation and movement, the change of film thickness of droplet mode with spray Reynolds was achieved by increasing impact frequency of droplet impact instead of the volume of a single droplet.

Experimental investigation on the dynamic behavior of small droplet in a uniform DC electric field

To study water droplet dynamic behavior in the silicone oil under a uniform DC electric field, a visual experiment system is set up. Two parallel plates are placed to form a uniform DC electric field. Results show when the electric field intensity is less than 2.33 kV/cm, with the increase of the electric field intensity, the droplet stretches in the vertical direction. The droplet detaches from the lower plate at 2.45 kV/cm. The droplet breaks into two daughter droplets at 3.67 kV/cm. Two daughter droplets coalesce at 4.67 kV/cm. A liquid bridge is formed at 4.93 kV/cm.