Prashant R. Waghmare

Contact Angle Hysteresis of Microbead Suspensions

Microbead suspensions are often used in microfluidic devices for transporting biomolecules. An experimental investigation on the wettability of microbead suspension is presented in this study. The variation in the surface tension and the equilibrium contact angle with the change in the volume fraction of the microbead is presented here. The surface tension of the microbead suspension is measured with the pendant drop technique, whereas the dynamic contact angle measurements, i.e., advancing and receding contact angles, are measured with the sessile drop technique. An equilibrium contact angle of a suspension with particular volume fraction is determined by computing an average over the measured advancing and receding contact angles. It is observed that the surface tension and the equilibrium contact angle determined from advancing and receding contact angles vary with the magnitude of the microbeads volume fraction in the suspension. A decrease in the surface tension with an increase in the volume fraction of the microbead suspension is observed. The advancement and the recession in contact line for dynamic contact angle measurements are achieved with the motorized dosing mechanism. For microbead suspensions, the advancement of the contact line is faster as compared to the recession of the contact line for the same flow rate. The presence of microbeads assists in the advancement and the recession of the contact line of the suspension. A decrease in the equilibrium contact angles with an increase in the microbead suspension volume fraction is observed. Inclusion of microbeads in the suspension increases the wetting capability for the considered combination of the microbead suspension and substrate. Finally, empirical correlations for the surface tension and the contact angle of the suspension as a function of microbead volume fraction are proposed. Such correlations can readily be used to develop mechanistic models for the capillary transport of microbead suspensions related to LOC applications.

Under-water superoleophobicity of fish scales

Recent surge in the development of superhydrophobic/superoleophobic surfaces has been motivated by surfaces like fish scales that have hierarchical structures, which are believed to promote water or oil repellency. In this work, we show that the under-water oil repellency of fish scales is entirely due to the mucus layer formation as part of its defense mechanism, which produces unprecedented contact angle close to 180°. We have identified the distinct chemical signatures that are responsible for such large contact angle, thereby making fish scale behave highly superoleophobic inside the water medium. In absence of the mucus layer, it is found that the contact angle decreases quite dramatically to around 150°, making it less oleophobic, the degree of such oleophobicity can then be contributed to its inherent hierarchical structures. Hence, through this systematic study, for the first time we have conclusively shown the role of the fishs mucus layer to generate superoleophobicity and negate the common notion that hierarchical structure is the only reason for such intrinsic behavior of the fish scales.

Amphiphobic surfaces from functionalized TiO2 nanotube arrays

Vertically-oriented, self-organized TiO2 nanotube arrays (TNAs) are a highly ordered n-type semiconducting nanoarchitecture with a wide range of potential applications. We generated low energy surfaces repellent to a broad spectrum of liquids by functionalizing TNAs using monolayers of two different fluorinated hydrocarbon molecules: perfluorononanoic acid (PFNA) and 1H, 1H′, 2H, 2H′-perfluorodecyl phosphonic acid (PFDPA). Nanotubes of two different outer diameters (50 nm and 130 nm) were studied and their wetting behavior analyzed in liquids belonging to different solvent classes to infer the nature of the wetting states. We show that the wetting behavior of perfluorinated monolayer-functionalized TNAs in polar liquids is explained by fakir or Cassie-states whilst the wetting behavior of bare nanotubes in every liquid is explained by Wenzel-type states. On the other hand, a transition between the Cassie and Wenzel states due to closed pores in the TNA architecture dictates the wetting behavior of functionalized TNAs in apolar liquids. The wetting behavior of functionalized TNAs is understood considering the synergistic effect of geometric and chemical surface modification. PFDPA-functionalized TNAs were found to be resilient to 24 hours of exposure to water and ethylene glycol at a static fluid pressure of 0.105 MPa, and are one step closer towards the realization of a mechanically robust omniphobic surface. At the same time, an understanding of wetting behavior will be useful in the design and optimization of a wide range of interface-sensitive devices such as metal oxide nanotube/nanopore array based sensors, implants, flow-through membranes, photocatalysts and heterojunction solar cells.

Modeling of combined electroosmotic and capillary flow in microchannels

In the present study, theoretical model for the transient response of a capillary flow under the combined effects of electroosmotic and capillary forces at low Reynolds number is presented. The governing equation is derived based on the balance among the electrokinetic, surface, viscous and gravity forces. A non-dimensional transient governing equation for the penetration depth as a function of time is obtained by normalizing the viscous, gravity and electroosmotic forces with surface tension force. A new non-dimensional group for the electroosmotic force, E(o), is obtained through the non-dimensional analysis. This new non-dimensional group is a representation of combined electroosmosis and surface tension, i.e., capillarity. The numerical solution of governing equation is obtained to study the effect of different operating parameters on the flow front transport. In a combined flow, it is observed that the flow with positive and low negative magnitude E(o) numbers, the attainment of equilibrium penetration depth is similar to a capillary flow. In case of high negative magnitude E(o) numbers, complete filling of the channel is observed. The electrolyte with lower permittivity delays the progress of the flow front whereas a large EDL transports the electrolyte quickly. Higher viscous and gravity forces also delay the transport process in the combined flow. This model suggests that in combined flow the electrokinetic parameters also play an important role on the capillary flow and experiments are required to confirm this electrokinetic effect on capillary transport.

Under-water superoleophobic Glass: Unexplored role of the surfactant-rich solvent

Preparing low energy liquid-repellant surfaces (superhydrophobic or superoleophobic) have attracted tremendous attention of late. In all these studies, the necessary liquid repellency is achieved by irreversible micro-nano texturing of the surfaces. Here we show for the first time that a glass surface, placed under water, can be made superoleophobic (with unprecedented contact angles close to 180 degrees and roll off angles only a few fractions of 1 degree) by merely changing the surfactant content of the water medium in which the oil (immiscible in water) has been dispersed. Therefore, we propose a paradigm shift in efforts to achieve liquid-repellant systems, namely, altering the solvent characteristics instead of engineering the surfaces. The effect occurs for a surfactant concentration much larger than the critical micelle concentration, and is associated to strong adsorption of surfactant molecules at the solid surface, triggering an extremely stable Cassie-Baxter like conformation of the oil droplets.

Finite reservoir effect on capillary flow of microbead suspension in rectangular microchannels

The present study reports a theoretical investigation of capillary transport of microbead suspension in microfluidic channels with finite size reservoirs at the inlet. The reservoir-microchannel combination is often the case in Lab-on-a-Chip (LOC) where biomolecules are transported using capillary force. To demonstrate such finite reservoir effect, the reservoir is placed vertically above the microchannel. Under such condition, the pressure field expression at the rectangular microchannel inlet is deduced. Appropriate correlations for effective physical properties are used to account for the presence of microbeads in the working fluid, mimicking biomolecules in actual LOC. The non-dimensional governing equation for capillary flow with finite size reservoir is derived based on the balance among the surface, viscous and gravity forces acting on the fluid front. The numerical solution of governing equation is obtained to investigate the impact of several operating parameters on the flow front progression. It is observed that the aspect ratio of the microchannel and reservoir play vital roles in deciding the behavior and magnitude of flow front progression in the microfluidic channels. Capillarity and gravity force dominant regions during the progression is observed. The microchannel width and reservoir width decide the interplay between gravity and capillarity. Although higher fluid level in the reservoir has an added advantage for more gravitational head, the resistance from the reservoir makes the flow front progress slowly at the beginning of the capillary transport. It is also found that microbead volume fraction in the working fluid plays an important role in delaying the capillary transport under various operating conditions. Hence, it can be concluded that the use of reservoir at the inlet of microfluidic channels has an impact on the overall capillary transport of biomolecules in LOC devices.

Dynamics of liquid droplets in an evaporating drop: liquid droplet “coffee stain” effect

In this paper, we demonstrate the dynamics of bidispersed oil droplets in an evaporating water sessile drop. This phenomenon is therefore equivalent to a unique liquid droplet based “coffee stain” effect, with the depositing colloidal particles (of a classical “coffee stain” problem) being replaced by the oil droplets partially wetting the substrate. The important difference with respect to the classical “coffee stain” problem, as revealed by our experiments, is that the oil droplets, unlike the colloidal particles, cannot reach the contact line; rather the aversion of the oil droplets to the air ensures that the oil droplets always remain at a finite distance from the contact line. We call this effect an “enclosure” effect, characterized by this distance. We provide a theoretical model to explain this phenomenon, and our theoretical results match well with the experimental observations. The “enclosure” effect depends on the droplet size, thereby allowing an automatic size-based separation of the oil droplets. Additionally, this effect depends on the wettability of the oil droplets and the sessile drop, as well as the relative velocity of the oil droplets with respect to the rate of decrease of the sessile drop contact angle. Our identification of this new phenomenon in a liquid-droplet based “coffee stain” problem will have a huge impact on microscale control and manipulation of liquid droplets in a two phase system.

Drop deposition on under-liquid low energy surfaces

In this paper, we propose a new technique for non-intrusive drop deposition on under-liquid low energy surfaces. This technique addresses the limitations of the conventional drop deposition method, in which a needle holding the drop is brought in close proximity to the solid substrate, and the drop gets deposited on the solid by detaching from the needle spontaneously owing to the favorable drop-substrate surface energy. Therefore, for low energy surfaces, irrespective of whether such surfaces are in air or under liquid, the fact that the drop-substrate surface energy is much smaller than the drop-needle surface energy, there are extreme difficulties in depositing the drop by getting it removed from the needle. In our proposed method, we address this limitation for the special case of under-liquid low energy surfaces. Under-liquid systems provide two distinct interfaces: first the surface–liquid interface where the drop is to be deposited and the second the liquid–fluid interface at the location where the liquid column ends. We achieve this non-intrusive, substrate-independent drop deposition method by depositing the drop from this liquid–fluid interface, by using the (un)favorable spreading behavior of the drop at this interface. Massive upsurge in use of low-energy under-liquid surfaces, requiring a correct estimation of the corresponding surface energy, makes our proposed technique extremely important and technologically significant.

Multifunctional reduced graphene oxide coated cloths for oil/water separation and antibacterial application

Multifunctional reduced graphene oxide (RGO) coated cloths were prepared through the thermal treatment of GO coated cloths without using any harmful reducing agent. The effect of heating temperature on the wetting properties of the cloths was also monitored through contact angle measurement. The highest contact angle 141° was observed for the RGO coated dense cloth while the highest contact angle exhibited by the RGO coated sparse cloth was 130°. The as prepared RGO coated cloths were utilized as a filter and an absorbent for oil/water separation. The RGO coated sparse cloth was utilized as a filter for the separation of an oil/water mixture while the dense cloth was utilized as an absorbent for the selective absorption of oil from water. RGO coated cloths were also explored as an antibacterial agent and the effect of the microstructure of coated cloths on their antibacterial properties was also investigated. The cloth with the dense structure exhibited higher antibacterial activity compared to the sparse cloth. A significant loss of percentage viability up to 98% was achieved using the RGO coated cloth, proving it highly efficient as an antibacterial agent, which makes it a suitable bandage material for the dressing of open wounds. Apart from being antibacterial, the RGO coated cloths are also hydrophobic in nature so they can protect wound from water and atmospheric moisture which is not possible with ordinary bandages.

Investigation of Combined Electro-Osmotic and Pressure-Driven Flow in Rough Microchannels

The present study is carried out to investigate the influence of surface roughness in combined electro-osmotic and pressure-driven flow in microchannel. Two-dimensional theoretical model is developed to predict the behavior of velocity profiles in rough microchannel. The concept of surface roughness-viscosity model is used to account the effect of surface roughness. The pluglike velocity profile for electro-osmotic flow and the parabolic velocity profile for pressure-driven flow with delay in attaining the centerline velocity are observed. It is found that for electro-osmotic flow, the deviation in velocity profile from a flow in a smooth channel occurs near the wall, whereas in pressure-driven flow, such deviation is dominant in the core region. A superposition of pluglike and parabolic velocity profiles is found in combined electro-osmotic and pressure-driven flow. It is also observed that in the case of combined flow, the deviation in velocity profile from the smooth channel case reduces gradually with the distance from the wall.