Droplet Mobility Research Articles

Electrophoresis of a highly charged dielectric fluid droplet in electrolyte solutions

Electrophoresis of a dielectric fluid droplet with arbitrary surface potentials is investigated theoretically in this study. A pseudo-spectral method based on Chebyshev polynomials is adopted to solve the governing electrokinetic equations. Droplet mobilities are expressed as functions of electrolyte strength for fluid droplets with various viscosities and surface potentials. Effects of various electrokinetic parameters upon the droplet motion are investigated extensively.It is found, among other things, that the viscosity-dependence of the droplet mobility is synchronized with the presence of a separate axisymmetric exterior vortex flow surrounding the droplet, in addition to the axisymmetric interior vortex flow induced by the spinning motion of the charged surface. With its presence, the more viscous a fluid droplet is, the faster it moves; otherwise, the viscosity-dependence is the other way around. Critical points are discovered for highly charged droplets, at which the droplet surface becomes immobile and the interior fluid becomes motionless, just like a rigid solid particle. The orientation of the interior vortex flow changes each time passing these critical points, accompanied by the onset or shutdown of the exterior vortex flow.The results are useful in various practical applications such as drug delivery.

Mechanistic studies of droplet electrophoresis: A review.

Electrophoresis (EP) of droplets is an intriguing phenomenon that has applications in biological systems, separation strategies, and reactor engineering. Droplet EP is significantly different from the classic particle EP because of droplet characteristics such as a mobile surface charge and the nonrigidity of the interface. Also, the liquid-liquid system, where there is an interplay between the hydrodynamic and electrokinetic forces in both phases, adds to the complexity of electrophoretic motion. Due to the vast amount of potential applications of droplet EP, a mechanistic understanding of the droplet motion in the presence of an external electric field is crucial. This review provides a background on the mechanism of droplet EP and summarizes the intrinsic interplay between the different relevant forces in these systems. The review also describes the key differences between droplet EP and particle EP, and the impact of these differences on droplet mobility. Additionally, we schematically summarize the effects of key parameters on droplet EP mobility, such as electric double layer polarization, the development of internal flow inside a droplet and boundaryeffects.

Dropwise condensation of low surface tension fluids on lubricant-infused surfaces: Droplet size distribution and heat transfer

The use of lubricant-infused surfaces and slippery liquid-infused porous surfaces to promote dropwise condensation of low surface tension fluids has received increasing attention in recent years due to the high condensate droplet mobility and low droplet contact angle hysteresis enabled by these coatings. Though multiple studies have focused on developing models to predict heat transfer during water vapor condensation on hydrophobic lubricant-infused surfaces, a lack of understanding exists for condensation of low surface tension fluids, where the surface becomes wettable to the condensate. In this study, we develop a theoretical model to predict the heat transfer during dropwise condensation of low surface tension fluids (ethanol and hexane) on lubricant-infused surfaces. Using numerical analysis, we develop a relationship between droplet Nusselt number and Biot number to accurately predict individual droplet heat transfer. The droplet size distribution of ethanol and hexane were experimentally determined, with result showing that the size distribution density depends on both the working fluid properties and droplet contact angle behavior. To validate our model, experiments were conducted by condensing pure ethanol and hexane vapor on different diameter tubes coated with lubricant-infused surfaces. From the experimental measurements, two correlations which characterized the distribution density of ethanol and hexane were developed. Using the droplet distribution correlations as input parameters, our developed model predicts the heat transfer coefficients for low surface tension fluid condensation more accurately when compared to the existing models, where the maximum deviation between the prediction and experimental results is less than 15%.

A comparison of bioinspired slippery and superhydrophobic surfaces: Micro-droplet impact

Slippery lubricant impregnated surfaces (SLIPSs/LISs) exhibit remarkable features of repellency and droplet mobility to a broad range of complex fluids. Their performance in micro-droplet repellency has received less attention. In this study, the anti-wetting performance of SLIPSs in comparison to superhydrophobic surfaces (SHSs) is investigated for the micro-droplet impact on different textured surfaces. Different series of square-pillar arrays are modeled to consider the effect of surface morphology on droplet hydrodynamics. A multiphase numerical model in conjunction with an accurate contact angle method has been implemented to analyze details of three immiscible phases during the droplet impact on the SLIPS. Our findings revealed that on the SLIPS with a low-density micro-textured surface where the effect of lubricant is more significant, droplet repellency and mobility are improved compared to SHSs. It was illustrated that on the SLIPS, droplet pinning decreased significantly and in low Weber number cases where the effect of lubricant is more noticeable, partial bouncing occurred. It was also observed that slippery surfaces with a low-density of micro-pillars exhibit bouncing behavior, which indicated the repellency effect of lubricant in droplet hydrodynamics. Although micro-droplets failed to recoil at a higher Weber number (We≃160) on both the SHS and the SLIPS, droplet penetration within the micro-structured surface was considerably smaller on the SLIPS.

Injectable cationic traps and sticky bacterial emulsifiers: A safe alliance during diesel bioremediation

This study proposes injectable polymeric filters to prevent diesel migration during its bioremediation. Pseudomonas aeruginosa (Gram negative bacterium) was isolated from forest soil and used to biodegrade diesel. P. aeruginosa produced biosurfactants that emulsified diesel in water while degrading it, by decreasing the interfacial tension and forming skins around diesel droplets. In lab experiments, emulsified diesel droplets could migrate through model soil columns (sand and limestone) saturated with water (to mimic aquifers). These results indicate that clean aquifers downstream of diesel spills are at risk of becoming contaminated during diesel bioremediation by P. aeruginosa. Injectable filters were developed to mitigate this risk. Injectable filters were obtained by injecting into model soils aqueous solutions of quaternized hydroxyethylcellulose (HEC+, with a positive electrostatic charge). HEC + adsorbed onto model soils, yielding positively charged surfaces. Diesel droplets stabilized by P. aeruginosa had a negative electrostatic charge (as demonstrated through zeta potential measurements) and adhered onto HEC + coated soil, likely due to attractive electrostatic forces. As a result, diesel droplet mobility through soil columns decreased. Our proof of concept indicates that injectable HEC + filters can protect clean regions of aquifers during the bioremediation of diesel spills by P. aeruginosa.

Study of spontaneous mobility and imbibition of a liquid droplet in contact with fibrous porous media considering wettability effects

In this paper, droplet mobility and penetration into a fibrous porous medium are studied considering different physical and geometrical properties for the fibers. An in-depth insight into the droplet imbibition into the fibrous medium is beneficial for improving membrane products in different applications. Herein, a multiphase lattice Boltzmann method is employed as an efficient numerical algorithm for predicting the multiphase flow characteristics and the interfacial dynamics affected by the interaction between the droplet and fibrous substrates considered. This computational technique is validated by comparison of the present results obtained for different benchmark two-phase flow problems with those reported in the literature, which shows good agreement and confirms its accuracy and efficiency. Droplet spreading and penetration into the fibrous porous geometries are then studied considering various porous topologies, intrinsic contact angles, and fiber sizes. This study shows that the intrinsic contact angle has a great influence on the capillary pressure and, consequently, on the droplet imbibition into the porous medium. The droplet easily penetrates the porous substrate by decreasing the intrinsic contact angle of the fibers, and vice versa. It is also concluded that by coating the fibrous porous medium with a narrow layer of thin fibers, the surface resistance to liquid penetration significantly increases. The present results illustrate that the droplet size impacts the directional wicking ability of the fibrous porous structure used in this study. This property should be considered to produce appropriate two-layer membranes for different applications.

Magnetic field based actuation and amalgamation of ferrofluid droplets on hydrophobic surface: An experimental and numerical study

This article presents a detailed experimental analysis along with numerical simulations to provide the information about shape evolutions and mechanism of actuation and amalgamation of the ferrofluid droplets deposited on a hydrophobic surface by moving a permanent magnet. To validate the numerical methods used in this article, a benchmark phenomenon of a sessile droplet spreading under the effect of a non-uniform magnetic field is first simulated, and the results are compared with available experimental observations. To further ensure the accuracy of experimental and numerical techniques and to understand the wetting properties and spreading behavior of non-stationary ferrofluid droplets, a prototype demonstration of the merging droplets on a hydrophobic solid surface in the presence of a permanent magnet is designed. It is observed that for hydrophobic surfaces, the merging droplets entrap an air bubble at the time of first contact. Moreover, the physics behind the transient variations of droplet morphology and the effect of the state-of-the-art parameters on droplet actuation are also discussed. The force evaluation, energy variations, velocity contours, and velocity vectors of the moving droplet are provided to understand the internal behavior of droplet mobility. Experiments are performed several times with different speeds of moving a magnet to find the critical velocity when the droplet fails to follow the magnet motion. While doing so, we encounter an anomalous phenomenon of thread formation and daughter droplet generation at the receding end of the sliding droplet. A phase diagram is also provided in the end, which describes different regions of the sliding phenomenon.

Topography-Dependent Effective Contact Line in Droplet Depinning.

The mobility of a fakir state droplet on a structured surface is fundamentally determined by the effective length of a microscopic contact line. However, it is largely unknown how the surface topography determines the effective contact line length. Based on the direct measurement of droplet adhesion force and the visualization of contact line, this work shows that effective contact line length is topography dependent as opposed to prior notion. On pored surfaces, contact line is not distorted, and the effective length approaches the droplet apparent perimeter regardless of pore dimensions. On pillared surfaces, the distortion of contact line is significantly dependent on the packing density of the pillar structures so that the effective length is as small as a pillar diameter on densely packed pillars and as large as a pillar perimeter on sparsely-packed pillars, while changing linearly between the two extremes.

Enhanced Condensation on Liquid-Infused Nanoporous Surfaces by Vibration-Assisted Droplet Sweeping.

Condensation is a universal phenomenon that occurs in nature and industry. Previous studies have used superhydrophobicity and liquid infusion to enable superior liquid repellency due to reduced contact angle hysteresis. However, small condensate droplets remain immobile on condensing surfaces until they grow to the departing size at which the body force can overcome the contact line pinning force. Hence, condensation heat transfer is limited by these remaining droplets that act as thermal barriers. To break these limitations, we introduce vibrational actuation to a slippery liquid-infused nanoporous surface (SLIPS) and show enhanced droplet mobility, controllable condensate repellency, and more efficient heat transfer compared to static SLIPSs. We demonstrate 39% smaller departing droplet size and 8× faster droplet departing speeds on the dynamic vibrating SLIPS compared to the nonactuated SLIPS. To understand the implications of these behaviors on heat transfer, we investigate the condensate area coverage and droplet distribution to verify enhanced dewetting on dynamic vibrating SLIPSs. Using well-validated heat transfer models, we demonstrate enhanced condensation heat transfer on dynamic SLIPSs due to the higher population of smaller condensate droplets (<100 μm). In addition to condensation heat transfer, we also show that vibrating SLIPSs can enhance droplet collection. This work utilizes the synergistic combination of surface chemistry and mechanical actuation to realize enhanced droplet mobility and heat transfer in an electrically controllable and switchable manner.

Surfactant-Controlled Mobility of Oil Droplets in Mineral Nanopores.

Polymer flooding is one of the widely used enhanced oil recovery (EOR) methods. However, tuning polymer properties to achieve improved performance in porous mineral rocks of diverse oil reservoirs remains one of the challenges of EOR processes. Here, we use molecular dynamics (MD) simulations to examine decane/water mixtures with surfactant additives in calcite and kaolinite mineral nanopores and characterize surfactant properties associated with increased fluid mobility and improved wettability in planar and constricted nanopore geometries. Cetyltrimethylammonium chloride (CTAC) and sodium dodecyl sulfate (SDS) surfactants are found to modulate the contact angles of decane droplets and reduce the decane density on mineral surfaces. CTAC can enhance and unblock the flow of decane droplets through narrowing nanopores with constricted geometries while aiding in decane droplet shape deformation, whereas SDS leads to decane droplets stalling in front of constrictions in nanopores. We hypothesize that the inability of the cationic CTAC headgroup to form hydrogen bonds is one of the key factors leading to enhanced CTAC-coated decane flow through constricted nanopores. The obtained molecular view of equilibrium and dynamic properties of complex fluids typical of oil reservoirs can provide a basis for the future design of new molecules for EOR processes.

Toward Passive Defrosting with Heterogeneous Coatings

A large amount of energy is lost in industrial active deicing. One problem is that water retention on the surface after ice melting decreases the overall heat-transfer coefficient by up to 20%. The Miljkovic group utilizes heterogeneous biphilic surfaces to enhance defrosting performance. They avoid water retention as the slush is rapidly drawn toward the hydrophilic regions before it completely melts.

Paperboard as a substrate for biocompatible slippery liquid-infused porous surfaces

Abstract Slippery liquid-infused porous surfaces or SLIPS were first introduced in 2011 by Wong et al. who reported a bioinspired self-repairing surface with remarkable slippery properties. Generally, production of these surfaces includes fossil-based or expensive materials and processes that are available mainly in laboratory scale. In this study, slippery surfaces with sliding angles of less than 10° are obtained using fibre-based material – paperboard – that is commercially available in large-scale and also cheap compared to substrates generally used in this field. The hierarchical nanostructure that is a necessary condition for appropriate droplet mobility was obtained by the liquid flame spray method. This method is fast, scalable, has a variety of optimization parameters and can be utilized in roll-to-roll technology that is traditional in paper industry. In this work, paperboard serves not only as a substrate, but also as a reservoir for the lubricant, thus it is important to evaluate the affinity of the material for the oils and estimate the capillary movement. Therefore, Cobb and Klemm methods were used when choosing a paperboard material. In addition to synthetic oils, rapeseed oil was also utilized as a lubricant, which potentially leads to eco-friendly and recyclable slippery liquid-infused porous surfaces.

Corrosion-Resistant Functional Diamond Coatings for Reliable Interfacing of Liquid Metals with Solid Metals.

Gallium-based liquid metals (GLMs) exist as atypical liquid-phase metals at and near room temperature while being electrically and thermally conductive, enabling copious applications in soft electronics and thermal management systems. Yet, solid metals are affected by interfacing with GLMs, resulting in liquid metal embrittlement and device failure. To avert this issue, mechanically durable and electrically tunable diffusion barriers for long-term reliable liquid metal-solid metal interfacing based on the deposition of various diamond coatings are designed and synthesized, as they feature high chemical inertness and extraordinary mechanical resistance. The diamond coatings show superlyophobicity (GLM contact angle ≥ 155°) and are nonstick toward GLMs, thereby achieving high mobility of GLM droplets (sliding angle 8-12°). The excellent barrier and anti-adhesion performance of the diamond coatings are proven in long-term experiments (3 weeks) of coated titanium alloy (Ti) samples in contact with GLMs. The electrical performance of the conductive diamond coating deposited on Ti is reliable and stable over a period of 50 h. As proof-of-concept applications a switch and a thermal management device based on liquid metals are demonstrated, signifying that coating diamond films on metals is a potent means to achieve stable integration of solid metals with GLMs.

Durability of superhydrophobic duplex coating systems for aerospace applications

In-flight icing, caused by the collision of supercooled water droplets with exposed aircraft surfaces, is an important safety hazard and a major issue in aviation. Superhydrophobic surfaces (SHS) have been shown to offer improvements to heating-based anti-icing and de-icing systems, reducing energy requirements for ice prevention and facilitating ice removal. A common SHS fabrication technique is to develop a surface with hierarchical roughness, and to then coat this surface with a hydrophobic topcoat. From this arises an issue: the durability of the entire system is fundamentally limited to the durability of this topcoat. In the present study, we develop a superhydrophobic duplex coating system with an emphasis on the environmental durability of the thin hydrophobic layer. The system consists of a thick TiO2 coating deposited by suspension plasma spraying, and a thin coating stack deposited by plasma enhanced chemical vapor deposition. The thin coating stack is based on DLC:SiOx—diamond-like carbon networked with silicon oxide—which exhibits a water contact angle of up to 95° and a hardness of up to 11 GPa, while the full coating system offers a contact angle of 159° and a contact angle hysteresis of 3.8°. The coating system is exposed to icing/deicing cycling, as well as rain erosion and accelerated aging tests; the results are compared with TiO2 coatings using stearic acid and fluoropolymer hydrophobic layers, as well as a commercially available superhydrophobic spray. The duplex coating system is shown to maintain water droplet mobility after 170 icing/deicing cycles, is resistant to prolonged UV and high-temperature exposure and offers a 300-fold improvement over the stearic acid in rain erosion tests.

The effect of III:V ratio on compositional and microstructural properties of GaAs1-xBix (0 0 1)

Compound semiconductors alloyed with Bi have many interesting properties, but the low solubility of Bi in these materials leads to effects such as Bi segregation and the formation of surface droplets. To study these effects, a series of GaAsBi films were grown as a function of growth conditions, namely the Bi:As and As:Ga flux ratios. Four surface morphologies emerged—surfaces that were free of droplets, contained Bi droplets, Ga droplets, or biphasic Ga/Bi droplets—as a function of flux ratio. Further analysis with X- ray diffraction, Transmission Electron Microscopy, and Atomic Probe Tomography revealed specific microstructures corresponding to the types of droplets present. Finally, we propose a model to explain each microstructure that takes into consideration factors including Ga adatom diffusion, bulk Bi diffusion, Bi droplet mobility, and film growth rate.

Interfacial Strategies for Smart Slippery Surfaces

The problem of contact line pinning on surfaces is pervasive and contributes to problems from ring stains to ice formation. Here we provide a single conceptual framework for interfacial strategies encompassing five strategies for modifying the solid-liquid interface to remove pinning and increase droplet mobility. Three biomimetic strategies are included, (i) reducing the liquid-solid interfacial area inspired by the Lotus effect, (ii) converting the liquid-solid contact to a solid-solid contact by the formation of a liquid marble inspired by how galling aphids remove honeydew, and (iii) converting the liquid-solid interface to a liquid-lubricant contact by the use of a lubricant impregnated surface inspired by the Nepenthes Pitcher plant. Two further strategies are, (iv) converting the liquid-solid contact to a liquid-vapor contact by using the Leidenfrost effect, and (v) converting the contact to a liquid-liquid-like contact using slippery omniphobic covalent attachment of a liquid-like coating (SOCAL). Using these approaches, we explain how surfaces can be designed to have smart functionality whilst retaining the mobility of contact lines and droplets. Furthermore, we show how droplets can evaporate at constant contact angle, be positioned using a Cheerios effect, transported by boundary reconfiguration in an energy invariant manner, and drive the rotation of solid components in a Leidenfrost heat engine. Our conceptual framework enables the rationale design of surfaces which are slippery to liquids and is relevant to a diverse range of applications.

Self-cleaning of glass surface to maximize the PV cell efficiency

Photovoltaic (PV) modules are widely used for harnessing solar energy which ensure maximum output when their glass surface is clean. However, PV modules are open to dust, grime and other contaminations which get deposited on their surface causing reduction in transmittance and hence their efficiency reduces. It is therefore required to clean the glass surface of PV modules time to time either manually by labor or using some special arrangements such as automated systems. However, these techniques are either laborious or require extra energy. Therefore, another solution to offset such complications is to use chemical coatings which ensure self-cleaning of glass surface by increasing water contact angle. In the present study, two types of water repellent chemicals (such as trimethylchlorosilane and hexamethyldisilazane) have been used to coat the glass surface using dip coating technique. The performance of such coated glass slides has been investigated using some important characterization techniques, such as finding transmittance by spectrophotometer and measuring water contact angle using a high resolution camera. Moreover, the self-cleaning effect has been observed using a microbalance to measure dust on coated glass exposed to open atmosphere and compared with uncoated glass. The results revealed that these coatings have increased the water contact angle up to 149% which reduces friction between the glass surface and water droplets. Moreover, the friction reduction helps in mobility of water droplets which in turn can easily carry out dust along with them, thus improving the efficiency of PV module.

Fabrication and performance of monodisperse liquid crystal droplet-based microchips for the on-chip detection of bile acids

Liquid crystal (LC) droplets have been widely used in the detection of chemical and biological species due to their large surface area-to-volume ratio and stimuli-responsive configuration transitions. However, the wide size distribution and high mobility of LC droplets in current LC droplet-based platforms have limited their real-time and quantitative detection applications. Here, we report a novel monodisperse LC droplet-based polydimethylsiloxane (PDMS) microchip, in which monodisperse LC droplets are in situ generated and trapped on a single microfluidic device. A flow-focusing element and a hydrodynamic-based microstructure are designed for the generation and entrapment of monodisperse LC droplets, respectively. This miniatured monodisperse LC droplet-based microchip containing a fixed number of monodisperse LC droplets allows us to quantitatively and rapidly (<4 min) detect bile acids in real time by monitoring the collapse of LC droplets under a polarized optical microscope with small sample consumption (~1.5 μL). The detection limits for cholic acid and deoxycholic acid are 10 μM and 1 μM, respectively. This study provides a way to develop simple and portable LC droplet-based sensing devices for the quantitative, rapid and real-time detection applications.

Free-Rising Bubbles Bounce More Strongly from Mobile than from Immobile Water-Air Interfaces.

Recently it was reported that the interface mobility of bubbles and emulsion droplets can have a dramatic effect not only on the characteristic coalescence times but also on the way that bubbles and droplets bounce back after collision (Vakarelski, I. U.; Yang, F.; Tian, Y. S.; Li, E. Q.; Chan D. Y. C.; Thoroddsen, S. T. Sci. Adv.2019, 5, eaaw4292). Experiments with free-rising bubbles in a pure perfluorocarbon liquid showed that collisions involving mobile interfaces result in a stronger series of rebounds before the eventual rapid coalescence. Here we examine this effect for the case of pure water. We compare the bounce of millimeter-sized free-rising bubbles from a pure water–air interface with the bounce from a water–air interface on which a Langmuir monolayer of arachidic acid molecules has been deposited. The Langmuir monolayer surface concentration is kept low enough not to affect the water surface tension but high enough to fully immobilize the interface due to Marangoni stress effects. Bubbles were found to bounce much stronger (up to a factor of 1.8 increase in the rebounding distance) from the clean water interface compared to the water interface with the Langmuir monolayer. These experiments confirm that mobile surfaces enhance bouncing and at the same time demonstrate that the pure water–air interfaces behave as mobile fluid interfaces in our system. A complementary finding in our study is that the ethanol–air interface behaves as a robust mobile liquid interface. The experimental findings are supported by numerical simulations of the bubble bouncing from both mobile and immobile fluid interfaces.

Condensate droplet size distribution and heat transfer on hierarchical slippery lubricant infused porous surfaces

In recent years, slippery lubricant infused porous surfaces (SLIPSs) have received important attention due to their excellent performance in applications such as condensation, low friction, self-cleaning and anti-icing, which is owed to the presence of an infused lubricant or oil effectively decreasing the liquid-solid interactions and enhancing droplet mobility when compared to hydrophobic and/or to superhydrophobic surfaces. In this work, we fabricate and make use of hierarchical micro-/nano-structured and nano-structured SLIPSs for condensation phase-change. Optical microscopy and macroscopic experimental observations are coupled to extract the droplet size distribution at different condensation times. Heat transfer resistance model through individual condensing droplets is further extended here to account for the presence of both micro- and nano-structures. Then, heat transfer through individual droplets is coupled to the droplet number density to estimate the heat transfer at different condensation times and their overall performance. A 100% greater heat transfer performance is reported on nano-structured SLIPSs when compared to hierarchical micro-/nano-structure SLIPSs due to the greater thermal resistance imposed by the micro-structures and the lubricant present within the structures. We conclude that although the presence of micro-structures shifts the droplet number density towards greater population of smaller sized droplets, this effect is not enough to overcome the greater heat transfer predicted on solely nano-structured SLIPSs. Findings presented here complement current research on SLIPSs and condensation phase-change heat transfer and are of great importance for the effective design of SLIPSs with enhanced condensation heat transfer performance.