arXiv subjects

Krishnacharya Khare

Publications and source records attributed to Krishnacharya Khare.

16 recordsLinked to original sources

arXiv2025

Study of surface roughness modulation on the adhesion behavior of PDMS elastomer

Adhesion control at the interface of two surfaces is crucial in various applications, including the design of micro- and nanodevices such as microfluidic devices, triboelectric nanogenerators, biochips, and electronic sensors. Several factors influence adhesion, including sample preparation, surface energy, mechanical properties such as modulus of elasticity, and surface texture or roughness. This study specifically investigates the effect of surface roughness on the adhesion behavior of the elastomer polydimethylsiloxane (PDMS), focusing on a complementary interface with identical roughness. To create surface roughness, sandpapers with grit sizes ranging from N = 120 to N = 2000 were used during the molding process. The surface roughness of the PDMS elastomer was then characterized using a stylus profilometer. Interfacial adhesion was evaluated through wedge test experiments, which enabled the analysis of the relationship between surface roughness, work of adhesion, and equilibrium crack length. Furthermore, the study also explores the correlation between the real area of contact and the work of adhesion.

cond-mat.soft↗

arXiv2025

Dynamics of Thin Lubricant Films upon Liquid Contact on Slippery Surfaces

In recent years, slippery surfaces have attracted significant interest due to their excellent liquid-repellent properties and their potential in diverse commercial applications. Such surfaces are prepared by coating functionalized solid substrates with a thin lubricant film that prevents direct contact between a liquid and the substrate. The morphology of thin films upon liquid contact plays a central role in governing various phenomena, including the coalescence and mobility of liquid droplets, heat transfer efficiency, and the extent of lubricant depletion. However, a detailed understanding of film dynamics upon droplet contact remains limited, both from theoretical and experimental perspectives. Here, by employing principles of fluid dynamics, optics, and surface wetting, we present a comprehensive study that examines both the spatial and temporal variations of lubricant films upon contact with sessile liquid droplets and liquid bridges. Our findings reveal that the film dynamics can be categorized into three distinct stages, each significantly influenced by key system parameters: initial film thickness, three-phase contact line width, and Laplace pressure of liquids. Furthermore, we demonstrate that by optimizing these parameters, it is possible to reverse the lubricant flow in the final stage, thereby causing the liquid to partially lift off from the slippery surface.

cond-mat.soft↗

arXiv2024

Adhesion study at the interface of PDMS-elastomer and borosilicate glass-slide

Adhesion control at the interface of two surfaces is crucial in many applications. Examples are the design of micro and nanodevices such as microfuidics devices, biochips, and electronic sensors. Adhesion at the interface of two materials can be controlled by various methods such as chemical treatment on the surface of the materials, modification of the surface texture of materials, and change of the mechanical properties of materials. The main idea of this study is to control the adhesion by changing the mechanical properties (modulus) of polydimethylsiloxane (PDMS) elastomer. We vary the modulus of PDMS elastomer by changing the mixing ratio of silicone elastomer base mixing ratio and its curing agent (Sylgard 184, Dow Corning). Our study also includes the effect of the thickness of the PDMS elastomer sheet on its adhesion behavior. Adhesion measurements at the interface of the borosilicate glass slide and different PDMS elastomer specimens were performed using a wedge test. This method inserts a glass coverslip at the interface to create the wedge. We observe a significant decrease in the work of adhesion and an increase in equilibrium crack length with an increase in elastic-modulus and thickness of the PDMS elastomer samples. We present and discuss the effect of modulus and specimen-thickness on the adhesion behavior of PDMS elastomer against glass slide.

cond-mat.soft↗

arXiv2023

Frequency Dependent Dewetting of Thin Liquid Films Using External ac Electric Field

Stability of thin liquid films on a surface can be controlled using an external stimuli, such as electric field, temperature or light by manipulating the total excess free energy of the system. It has been previously shown that thin lubricating films on slippery surfaces can be destabilized via spinodal mechanism using external electric field, which return to the original stable configuration upon switching off the electric field. However, the role of the frequency of the applied electric field is not clear, which is the main topic of study in this report. When an ac electric field of fixed voltage and varying frequency is applied across thin lubricating films of slippery surfaces, different dewetting behavior is observed. Characteristic length and time scales of dewetting depend strongly on the frequency of the applied voltage, which is primarily due to the change in the dielectric behavior of the lubricating fluid. In addition, the interplay of various time scales involved in the dewetting process also depend on the frequency.

cond-mat.soft↗

arXiv2023

Effect of externally deposited nanoscale heterogeneities in thin polymer films on their adhesion behavior

Adhesion between two surfaces depend on the chemical and the mechanical properties of both the materials. However, heterogeneities (surface or bulk) affect the adhesion between the two surfaces tremendously. In this work, we study the role of externally deposited nanoscale heterogeneities on their adhesion behavior. Silica nanoparticles in the bulk polydimethylsiloxane (PDMS) polymer matrix act as external heterogeneities, which subsequently affect their adhesion. The nanoscale heterogeneities change the polymer environment locally, which subsequently modify its mechanical properties, e.g. elastic modulus, as a result, it affects its adhesion behavior. We observe that the adhesion behavior vary in nonlinear manner with increasing nanoparticle concentration. Therefore, probing these heterogeneities may allow us to understand the emergence of processing-induced deviations and the role of external defects in the macroscopic properties of a polymer.

cond-mat.soft↗

arXiv2022

Numerical and Experimental Investigation of Static Wetting Morphologies of Aqueous Drops on Lubricated Slippery Surfaces Using a Quasi-Static Approach

Due to the slow dynamics of the wetting ridge, it is challenging to predict the wetting morphology of liquid drops on thin lubricant coated surfaces. It is hypothesized that when a drop sinks on a lubricated surface, quasi-static wetting morphology can be numerically computed only from the knowledge of interfacial energies, lubricant thickness, and drop volume. We used Surface Evolver software for the numerical computation of the interface profiles for a four-phase system. For the experiments, we used drops of 80 wt% formamide on silicone oil coated substrates with varying lubricant thickness, substrate wettability and drop volume. Optical images of drops were used to compare the experimental interfacial profiles and apparent contact angles with the numerically computed ones. We found good agreement between the experiments and the simulations for the interfacial profiles and apparent contact angles as a function of various systems parameters except for very thin lubricating films. Apparent contact angles varied non-linearly as a function of substrate wettability and lubricant thickness, however, were found constant with the drop volume.

cond-mat.soft↗

arXiv2022

A Simple Electronic Circuit Demonstrating Hopf Bifurcation for an Advanced Undergraduate Laboratory

A nonlinear electronic circuit comprising of three nodes with a feedback loop is analyzed. The system has two stable states, a uniform state and a sinusoidal oscillating state, and it transitions from one to another by means of a Hopf bifurcation. The stability of this system is analyzed with nonlinear equations derived from a repressilator-like transistor circuit. The apparatus is simple and inexpensive, and the experiment demonstrates aspects of nonlinear dynamical systems in an advanced undergraduate laboratory setting.

nlin.AO↗

arXiv2022

Reversible Dewetting of Thin Lubricating Films Underneath Aqueous Drops Using External Electric Field

The stability of thin liquid films on a surface depends on the excess free energy of the system involving various short-range and long-range interactions. In an unstable condition, thin liquid films may dewet into multiple small-sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using an external stimulus, one can control the stability of thin liquid films on demand. Here, we study the reversible dewetting process of thin lubricating films underneath aqueous drops on slippery surfaces using an external electric field. Upon applying voltage, stable thin lubricating films dewet in a manner identical to the spinodal dewetting of nanometer-thick liquid films. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, however the time taken during rewetting is very different compared to dewetting. The characteristic features of both the dewetting and rewetting processes are present over multiple cycles. Due to the random nature of the spinodal dewetting process, the final dewetting pattern does not show any correlation over multiple dewetting cycles.

cond-mat.soft↗

arXiv2021

Dewetting of Thin Lubricating Films Underneath Aqueous Drops on Slippery Surfaces

Stability of thin lubricating fluid coated slippery surfaces depends on the surface energy of the underlying solid surface. High energy solid surfaces, coated with thin lubricating oil, lead to the dewetting of the oil films upon depositing aqueous drops on it. The total surface energy, which is due to the long range and short range interactions, also predict the instability of thin lubricating films under the given condition. In this article, we present experimental study of dewetting of thin lubricating oil films sandwiched between hydrophilic solid surface and aqueous drops. Fluorescence imaging of lubricant film and wetting behavior of aqueous drops are used for the analysis. We find that the dewetting dynamics and the final pattern depend strongly on the thickness of the lubricating oil film.

cond-mat.soft↗

arXiv2020

Anisotropic Stick-Slip Behavior of Aqueous Drops on Lubricated Chemically Heterogeneous Slippery Surfaces

Conventional slippery surfaces show isotropic drop mobility in all directions, but many applications require directional drop motion along a particular path only. In previous studies, researchers used topographic substrates, together with different external stimuli, to demonstrate anisotropic drop motion, which is not very efficient and cost-effective. Herein, we report a novel approach to smartly control drop motion on lubricating fluid coated chemically heterogeneous surfaces composed of alternating hydrophobic and hydrophilic stripes. Upon depositing an aqueous drop on such a surface, the underneath lubricating fluid dewets from the hydrophilic regions but remains intact on the hydrophobic ones, providing sticky and slippery areas for the drop. This results in remarkable anisotropic drop sliding behavior, from uniform motion along parallel to stripes to stick-slip motion along the perpendicular to them. Furthermore, we also demonstrate a phase diagram summarizing different dynamic situations exhibited by drops, sticking, or moving in one or both directions.

cond-mat.soft↗

arXiv2016

Uniting Superhydrophobic, Superoleophobic and Lubricating Fluid Infused Slippery Behavior on Copper Oxide Nano-structured Substrates

Copper oxide nanostructures with spherical (0D), needle (1D) and hierarchical cauliflower (3D) morphologies are used to demonstrate superhydrophobic, superoleophobic and slippery behavior. These nanostructures are synthesized on galvanized steel substrates using a simple chemical bath deposition method by tuning precursor concentration. Subsequent coating of low surface energy polymer, polydimethylsiloxane, results in superhydrophobicity with water contact angle ~160(2)° and critical sliding angle ~2°. When functionalized with low-surface energy perfluoroalkyl silane, these surfaces display high repellency for low surface tension oils and hydrocarbons. Among them, the hierarchical cauliflower morphology exhibits better re-entrant structure thus show the best superoleophobicity with 149° contact angle for dodecane having surface tension 25.3 mNm-1. If these nanostructured substrates are infused with lubricant Silicone oil, they show excellent slippery behavior for water drops. Due to the lubricating nature of Silicone oil, the Silicone oil infused slippery surfaces (SOIS) show low contact angle hysteresis (~2°) and critical tilt angle (~2°). The hierarchical cauliflower nanostrcuture exhibit better slippery characteristics and stability compared to the other nanostructured surfaces.

cond-mat.soft↗

arXiv2016

Enhanced Slippery Behavior and Stability of Lubricating Fluid Infused Nanostructured Surfaces

Stability of lubricating fluid infused slippery surfaces is a concern for scientists and engineers and attempts are being made for its improvement. Lubricating oil coated slippery surface for aqueous drops is one of the important candidates in this class and their stability needs be improved to make them useful for practical applications. Cloaking of water drops with thin lubricant layer results in the loss of lubricant leading to deterioration of slippery behavior. Surface roughness or porosity provides larger surface area to the lubricating fluid and would to affect the stability of the lubricating film. Here we report the effect of surface roughness, from tens of nanometer to few microns, on the stability of slippery surface. Samples with small nanoscale roughness show improved performance in terms of contact angle hysteresis, critical tilt angle and slip velocity. Whereas large roughness samples show poorer performance compared to small nanoscale roughness and smooth samples. Small nanoscale roughness samples also show relatively slower deterioration against loss of lubricant during water flow. Once completely lost, the slippery behavior can be restored again simply by coating the sample again by the lubricating fluid.

cond-mat.soft↗

arXiv2016

Mechanically Tunable Slippery Behavior on Soft Poly(dimethylsiloxane) (PDMS) Based Anisotropic Wrinkles Infused with Lubricating Fluid

We demonstrate a novel technique to fabricate mechanically tunable slippery surfaces using one dimensional (anisotropic) elastic wrinkles. Such wrinkles show tunable topography (amplitude) on the application of mechanical strain. Following Nepenthes pitcher plants, lubricating fluid infused solid surfaces show excellent slippery behavior for test liquid drops. Therefore combining the above two i.e. infusing suitable lubricating fluid on elastic wrinkles would enable us to fabricate mechanically tunable slippery surfaces. Completely stretched (flat) wrinkles have uniform coating of lubricating fluid whereas completely relaxed (full amplitude) wrinkles have most of the lubricating oil in the wrinkle grooves. Therefore water drops on completely stretched surface show excellent slippery behavior whereas on completely relaxed surface they show very poor slippery behavior. Therefore continuous variation of wrinkle stretching provide reversibly tunable slippery behavior on such system. Since the winkles are one dimensional, they show anisotropic tunability of slippery behavior depending upon whether test liquid drops slip parallel or perpendicular to the wrinkles.

cond-mat.soft↗

arXiv2015

Investigation of slippery behaviour of lubricating fluid coated smooth hydrophilic surfaces

In the recent years many research groups have studied slippery properties on lubricating fluid infused rough surfaces using hydrophobic substrates. These surfaces show excellent slippery behaviour for water and other liquids. Here we demonstrate a simple method to fabricate stable slippery surfaces based on silicone oil coated hydrophilic samples. At room temperature, as prepared samples exhibit non-slippery behaviour due to sinking of water drops inside silicone oil layer because of inherently hydrophilic silicon substrate. Subsequent annealing at higher temperatures provides covalent bonding of silicone molecules at silicon surface making the surface hydrophobic which was confirmed by lubricant wash tests. So the silicone oil coated annealed samples show excellent water repellency, very low contact angle hysteresis and very good slippery behavior. But these surfaces show poor oil stability against drops flow due to cloaking of the oil around water drops which can be prevented by using drops of larger volume or continuous flow of water.

cond-mat.soft↗

arXiv2015

Reversible Transition between Superoleophobic and Superoleophilic States on Titania Coated Substrates by UV Irradiation

We demonstrate that tunable superoleophobic surfaces fabricated by a simple spin and spray coating methods of titania on silicon (Si) wafers and stainless steel (SS) mesh, which possess a hierarchical re-entrant structure consisting of nano meter sized particles on top of micron sized particle, are able to induce superoleophobicity on an oleophilic self assembled monolayer of 1H,1H,2H,2H-perflurodecyltrichlorosilane (FDTS). Though comparison between different coating methods, we show that spray coating on Si substrate and SS mesh can repel lower surface tension liquids than spin coating on Si substrates due to formation of half spherical micro-particles on the spray coated substrates which is confirmed by FESEM images. Subsequently, superoleophobic surfaces changes its super repellent property to complete wetting of any liquids under UV illumination by decomposing the FDTS molecules via photo catalytic property of titania particles. The superoleophobic property was regained by annealing followed by grafting of FDTS on the UV treated surface and the total process is repeatable over number of cycles.

cond-mat.soft↗

arXiv2015

Electrowetting on dielectrics on lubricating fluid based slippery surfaces with negligible hysteresis

Low voltage electrowetting on dielectrics on substrates with thin layer of lubricating fluid to reduce contact angle hysteresis is reported here. On smooth and homogeneous solid surfaces, it is extremely difficult to reduce contact angle hysteresis (contact angle difference between advancing and receding drop volume cycle) and the electrowetting hysteresis (contact angle difference between advancing and receding voltage cycle) below 10°. On the other hand, electrowetting hysteresis on rough surfaces can be relatively large (>30°) therefore they are of no use for most of the fluidic devices. In the present report we demonstrate that using a thin layer of dielectric lubricating fluid on top of the solid dielectric surface results in drastic reduction in contact angle hysteresis as well as electrowetting hysteresis (< 2°) on smooth as well as rough surfaces. Subsequently fitting the Lippmann-Young electrowetting equation to the experimental electrowetting data reveal that the dielectric lubricating fluid layer is only responsible for smooth movement of the three phase contact line of the liquid drop and does not affect the effective specific capacitance of the system.

cond-mat.soft↗