Hydrophilic Strip Research Articles

Coalescence and Rebound Dynamics in Two Droplets Train Impacting on a Heterogeneous Wettability Surface.

Droplets that can be steered and rebound off surfaces are fundamentally interesting and important due to their promising potential in numerous applications, such as anti-icing and -fogging, spray coating, and self-cleaning. Heterogeneous wettability surfaces have been shown to be an effective means of droplet manipulation. This paper combines numerical simulation with theoretical analysis to investigate the dynamics of two droplets training impacting on and bouncing off a heterogeneous surface (superhydrophobic substrate decorated with a hydrophilic strip). First, the time evolutions of the droplet morphology and velocity vectors are examined to explore the particular dynamic behaviors. At different ratios of the impact velocity, three distinct rebound patterns are observed, and a regime diagram is established. After that, the effects of the impact conditions and surface wettability on the rebound performance of the coalesced droplet are studied systematically. Special attention is paid to the variations of the rebound height and the lateral transportation distance with the Weber number of two droplets and the distance between the impacting point and the hydrophilic stripe. Moreover, a theoretical analysis of two droplets' impact is performed based on the energy conservation. The obtained scaling laws match well with the numerical data in the trend. Our research may strengthen the understanding of the interactions between droplets, which is valuable for the manipulation of multiple droplets in related fields.

Lattice boltzmann investigation of droplet interactions with non-uniform chemically patterned surfaces

In this study, we employed a non-orthogonal, three-dimensional, multi-relaxation-time pseudo-potential lattice Boltzmann method, and investigate the behaviors of droplets impacting chemically patterned surfaces. We considered two interfaces: a hydrophobic/neutral strip on a hydrophilic wall (surface A), and a hydrophilic strip on a hydrophobic wall (surface B). The dynamics of impact, including the evolution of the morphology and the droplet spreading factor, were investigated under the influence of the difference in wettability, Weber number (We), and strip width. An increase in the wettability difference of surface A delayed droplet detachment and reduced the amplitude of oscillations, where this can be attributed to the surface tension and viscous dissipation, which had more time to weaken the strength of the jet. The magnitude of droplet detachment time initially increased with We but eventually decreased. As We is further increased, the ratio of viscous loss to the initial kinetic energy of the droplet is decreased and resulted in a shorter detachment time. The unbalanced Young's force significantly affected the evolution of the droplet on surface B. The mass of the droplet accumulated near the borderline of the strip and expanded along the y-axis under the influence of the inertial force, where this led to a larger spreading factor along the y-axis. In addition, the mass of the droplet on the hydrophobic wall affected the strength of the unbalanced Young's force. As the strip width increased, the spreading factor initially increased but then decreased along the y-axis owing to the combined action of inertial forces.

Investigation of Coalescence-Induced Droplet Jumping on Mixed-Wettability Superhydrophobic Surfaces

Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width.

A Novel Wick-Like Paper-Based Microfluidic Device for 3D Cell Culture and Anti-Cancer Drugs Screening.

Paper is increasingly recognized as a portable substrate for cell culture, due to its low-cost, flexible, and special porous property, which provides a native cellular 3D microenvironment. Therefore, paper-based microfluidics has been developed for cell culture and biomedical analysis. However, the inability of continuous medium supply limits the wide application of paper devices for cell culture. Herein, a paper-based microfluidic device is developed with novel folded paper strips as wick-like structure, which is used for medium self-driven perfusion. The paper with patterns of hydrophilic channel, culture areas, and hydrophobic barrier could be easily fabricated through wax-printing. After printing, the hydrophilic paper strip at the periphery of the lower layer is then folded at 90° and extended into the medium container for continuous automatic supply of medium to the cell culture area. Tumor cells cultured in the paper device are tested for anti-cancer drug screening. Visualized cell viability and chemical sensitivity testing can be achieved by colorimetry combined with simple smartphone imaging, effectively reducing precision instrument dependence. The wick paper-based microfluidic device for cell culture endows the method the advantages of lower cost, ease-of-operation, miniaturization, and shows a great potential for large-scale cell culture, antibody drug production, and efficient screening.

Paper-Based Batteries as Conductivity Sensors for Single-Use Applications.

We present a novel approach to measure ionic conductivity with a self-powered strategy. In particular, we propose the use of a paper-based battery as a sensor. The battery sensor unit consists of two electrodes placed side-by-side and covered by a piece of hydrophilic paper strip. The electrodes are externally connected to a resistive element. The addition of the fluid to be sensed—which acts as the electrolyte—activates the battery, which generates an output voltage that is dependent on the conductivity of the liquid sample. The device, which is conceived as a single-use disposable sensor, has been tested with different synthetic and biological liquid samples. The battery sensor effectiveness has been assessed by comparing its performance with a commercial laboratory conductometer. The device opens new avenues for conductivity monitoring in small portable and wearable devices, as it simplifies the number of electronic components and the need of additional power sources.

Collection of nanoparticles at the air-liquid interface by surface tension gradients

Particles such as nano-plastics at the air-liquid interface is harmful to the environment. Due to small size of the nanoparticles, expensive materials are needed to collect them such as ultrafiltration membranes. In our study, we propose an easy method to collect the nanoparticles at the air-liquid interface. Because surface tension of contaminated water is lower than that of deionized water, the surface tension gradient is used to transport the nanoparticles. In our study, we wet a strip with deionized water and then immerse one end of the wetting strip into contaminated water. Due to the surface tension gradient between the contaminated water and the deionized water, the nanoparticles at the surface of the contaminated water are transported to the surface of the strip. In addition, by adding a small amount of deionized water at the other end of the strip, the nanoparticles can be constantly collected. Furthermore, many materials can be used as the hydrophilic strip, such as glass, paper, and steel mesh.

Hydrophilic strips for preventing air bubble formation in a microfluidic chamber.

In a microfluidic chamber, unwanted formation of air bubbles is a critical problem. Here, we present a hydrophilic strip array that prevents air bubble formation in a microfluidic chamber. The array is located on the top surface of the chamber, which has a large variation in width, and consists of a repeated arrangement of super- and moderately hydrophilic strips. This repeated arrangement allows a flat meniscus (i.e. liquid front) to form when various solutions consisting of a single stream or two parallel streams with different hydrophilicities move through the chamber. The flat meniscus produced by the array completely prevents the formation of bubbles. Without the array in the chamber, the meniscus shape is highly convex, and bubbles frequently form in the chamber. This hydrophilic strip array will facilitate the use of a microfluidic chamber with a large variation in width for various microfluidic applications.

Elastocapillary snapping: capillarity induces snap-through instabilities in small elastic beams.

We report on the capillarity-induced snapping of elastic beams. We show that a millimeter-sized water drop gently deposited on a thin buckled polymer strip may trigger an elastocapillary snap-through instability. We investigate experimentally and theoretically the statics and dynamics of this phenomenon and we further demonstrate that snapping can act against gravity, or be induced by soap bubbles on centimeter-sized thin metal strips. We argue that this phenomenon is suitable to miniaturization and design a condensation-induced spin-off version of the experiment involving a hydrophilic strip placed in a steam flow.

Hydrodynamics of the developing region in hydrophobic microchannels: A dissipative particle dynamics study

Dissipative particle dynamics (DPD) is becoming a popular particle-based method to study flow through microchannels due to the ease with which the presence of biological cells or DNA chains can be modeled. Many lab-on-a-chip devices require the ability to manipulate the transport of cells or DNA chains in the fluid flow. Microchannel surfaces coated with combinations of hydrophilic and hydrophobic materials have been found useful for this purpose. In this work we study numerically the hydrodynamics of a steady nonuniform developing flow between two infinite parallel plates with hydrophilic and hydrophobic surfaces using the DPD. The hydrophobic and hydrophilic surfaces are modeled using partial-slip and no-slip boundary conditions, respectively, in the simulations. We also propose a method to model the inflow and outflow boundaries for the DPD simulations. The simulation results of the developing flow are in good agreement with analytical solutions from continuum theory for no-slip and partial-slip surfaces. The entrance region constitutes a considerable fraction of the channel length in miniaturized devices. Thus it is desirable for the length of the developing region to be short as most microfluidic devices such as cell or DNA separators and mixers are designed for the developed flow field. We study the effect of a hydrophilic strip near the inlet of a hydrophobic microchannel on the developing length. We find that the presence of the hydrophilic strip significantly reduces the developing length.

Investigation of hydrophilic modification for bubble-free operation in microfluidic systems and micropump applications

This study developed a bubble-free method for microfluidic devices and micropump applications by modifying the wetting characteristic of their chamber surfaces. Two methods of hydrophilic film formation were investigated, that is, microwave plasma surface modification and TiO 2 thin film deposition. The evaluation results indicated that TiO 2 thin film deposition showed better stability and it was therefore selected to improve the surface wettability and unify the spreading behavior. Different hydrophilic strip shape and strip numbers were investigated and the results indicated that the vertical design with trisection strip gives the best result and effectively discharges the bubbles of microfluidic devices. The results were then applied to a peristaltic micropump and very good results were obtained. That is, the micropump stability and robustness are enhanced significantly. Furthermore, in the pump operation frequency range (i.e. 75 ±10 Hz), bubbles are discharged effectively. The results show that when the micropump operated at frequencies lower than 100 Hz, air bubbles became insignificant; therefore, operation frequencies lower than 100 Hz are considered to be the micropump's stable performance range. From the results it was concluded that bubble formation is also responsible for the flow rate downhill effect.

Capillary channels in a gravitational field

We investigate the shape of the liquid–air free interface in the next two settings. First, we insert two parallel vertical plates sufficiently close in an infinite reservoir of liquid. Due to capillary and gravity forces and when the equilibrium is achieved, the liquid rises to a certain height. Then the liquid–air interface meets the vertical walls at prescribed angles and its mean curvature is a linear function of the height, that is, of the coordinate function that defines the gravity field. We study the shapes of these interfaces and their qualitative properties assuming natural hypothesis on symmetry. One matter of interest is to obtain estimates of the size of the capillary meniscus, such as its height, in terms of the boundary data. In the second setting, we consider a horizontal hydrophilic strip surrounded by a solid region of hydrophobic character. We spread the liquid over the strip driven by wettability in such a way that the liquid remains confined up to the boundary of the strip. In a state of equilibrium and assuming that the liquid is invariant in the direction of the non-bounded coordinate of the strip, we prove results on the existence and the uniqueness and we analyse the behaviour of the interface, specially related to the estimates of volume enclosed by the surface. Both settings are particular situations of the one-dimensional case of the capillarity problem, which has been studied in the literature to describe the shape of a liquid that faces a vertical plate.

A Simplified Model for V-ATPase<tex>$hboxH^+$</tex>Extrusion

An analytical model of V-type H+-translocating ATPase (V-ATPase) was developed based on an approximation to the mechanochemical model of Grabe et al. (Biophys. J., pp. 2798-2813, vol. 78, 2000). Grabe's work utilizes structural information and physiological assumptions to construct a detailed mechanochemical model of the V-ATPase. Due to the complexity of their model, it does not give a readily usable mathematical expression for the V-ATPase current. Based on their analysis of the structure of the proton pump, we develop a two-compartment model of the V-ATPase, which contains a membrane "half-channel" for proton translocation separated by a hydrophilic strip and a hydrophobic wall from the cytoplasm. Using the Langevin equation to describe proton transport across the membrane, we simplify the model based on their assumptions on the molecular structure of the pump and arrive at a general form of solution to the proton pump flux driven by ATP hydrolysis based on assumptions on the physiological properties of the strip and the wall, as well as the two fluid compartments. In this process of simplification, we explicitly relate V-ATPase structure, stoichiometry, pump efficiency, and ATP hydrolysis energy to the active pump current. The simplified model is used to provide model-generated approximations to measured data from a variety of laboratories. In addition, it provides a very compact characterization of V-ATPase, which can be used as a proton extruder in a variety of different cell membranes, as well as in the membranes of intracellular organelles. Index Terms-Electrophysiology, mechanochemstry, molecular motors, proton extrusion

Late stage kinetics for various wicking and spreading problems.

The kinetics of spreading of a liquid drop in a wedge or V-shaped groove, in a network of such grooves, and on a hydrophilic strip, is reexamined. The length of a droplet of volume Omega spreading in a wedge after a time t is predicted to scale as Omega(1/5) t(2/5), and the height profile is predicted to be a parabola in the distance along the wedge. If the droplet is spreading radially in a sparse network of V-shaped grooves on a surface, the radius is predicted to scale as Omega(1/6) t(1/3), provided the liquid is completely contained within the grooves. A number of other results are also obtained.

A Capillary Electrophoresis Conductometric Detector Based on Measurement of the Conductivity of a Plastic Hydrophilic Strip

A Capillary Electrophoresis Conductometric Detector Based on Measurement of the Conductivity of a Plastic Hydrophilic Strip