Abhiram Hens

Analysis of interfacial instability and multimode bubble formation in saturated boiling using coupled level set and volume-of-fluid approach

The dynamics of vapor-liquid interface are important because interfacial instability determines bubble growth, detachment frequency, waiting time, shape of bubbles, and the interrelationship between bubble formation sites. In this study, a detailed numerical simulation has been performed to understand the transition in bubble release pattern and multimode bubble formation in saturated pool boiling. The interfaces drop down alternatively at the nodes and antinodes of the wavelengths dictated by Rayleigh-Taylor instability and Taylor-Helmholtz instability. Due to higher degrees of superheat, vapor jets emanate from nodes and antinodes. An attempt has been made to predict the maximum and minimum heat fluxes during saturated pool boiling.

Vapor-liquid phase coexistence and transport properties of two- dimensional oligomers

Grand-canonical transition-matrix Monte Carlo and histogram reweighting techniques are used herein to study the vapor-liquid coexistence properties of two-dimensional (2D) flexible oligomers with varying chain lengths (m = 1-8). The phase diagrams of the various 2D oligomers follow the correspondence state (CS) principle, akin to the behavior observed for bulk oligomers. The 2D critical density is not influenced by the oligomer chain length, which contrasts with the observation for the bulk oligomers. Line tension, calculated using Binders formalism, in the reduced plot is found to be independent of chain length in contrast to the 3D behavior. The dynamical properties of 2D fluids are evaluated using molecular dynamics simulations, and the velocity and pressure autocorrelation functions are investigated using Green-Kubo (GK) relations to yield the diffusion and viscosity. The viscosity determined from 2D non-equilibrium molecular dynamics simulation is compared with the viscosity estimated from the GK relations. The GK relations prove to be reliable and efficient for the calculation of 2D transport properties. Normal diffusive regions are identified in dense oligomeric fluid systems. The influence of molecular size on the diffusivity and viscosity is found to be diminished at specific CS points for the 2D oligomers considered herein. In contrast, the viscosity and diffusion of the 3D bulk fluid, at a reduced temperature and density, are strongly dependent on the molecular size at the same CS points. Furthermore, the viscosity increases and the diffusion decreases multifold in the 2D system relative to those in the 3D system, at the CS points.

Pathways from disordered to ordered nanostructures from defect guided dewetting of ultrathin bilayers.

Transitions from spinodal to pattern-guided dewetting of a bilayer of ultrathin films (<10nm) confined between a pair of patterned surfaces have been explored employing molecular dynamic (MD) simulations. The physical or chemical defects of different sizes and shapes are decorated on the confining substrates by either removal or addition of multiple layers of similar or dissimilar atoms. The simulations are performed to identify the transition from spinodal pathway to the heterogeneous nucleation route, with the variation in the size of the substrate patterns. The MD simulations reveal the limits beyond which the defects can guide the dewetting to generate ordered patterns of nanoscopic size and periodicity. Comparing the results obtained from the MD simulations with the more widely employed continuum dynamics approach highlights the importance of the MD approach in quantitatively analyzing the dynamics of the dewetting of ultrathin films. The study demonstrates that the pattern-guided dewetting of confined bilayers can lead to ordered holes, droplets, and stripes with size and periodicity less than 10nm, which are yet to be realized experimentally and can be of significance for a number of future applications.

Evaporation of water droplets on Pt-surface in presence of external electric field—A molecular dynamics study

Evaporation of a sessile droplet on a hot solid substrate is an important problem in fluid mechanics. It is relevant to theoretical issues in heat transfer as well as several practical applications. This study investigates the spreading and evaporation of a nanoscale water droplet on a solid platinum surface. The major objective was to analyze the effect of an external electric field on these phenomena. Varying the intensity and direction of the external electric field, a series of molecular dynamics simulations were carried out to understand these phenomena at a molecular level. The results reveal that a horizontal electric field assists in droplet spreading, whereas a vertical electric field enhances the rate of evaporation for a certain range of field intensities. It also shows that the substrate temperature plays an important role in such processes. It is seen that the effect of an external electric field on droplet evaporation becomes significant at an intermediate range of surface temperatures and this effect is not clearly visible for either very high or very low range of surface temperatures.

A novel approach to fabricate dye-encapsulated polymeric micro- and nanoparticles by thin film dewetting technique

A new method is reported for fabrication of polymeric micro- and nanoparticles from an intermediate patterned surface originated by dewetting of a polymeric thin film. Poly (d, l-lactide-co-glycolide) or PLGA, a biocompatible polymer is used to develop a thin film over a clean glass substrate which dewets spontaneously in the micro-/nano-patterned surface of size range 50nm to 3.5µm. Since another water-soluble polymer, poly vinyl alcohol (PVA) is coated on the same glass substrate before PLGA thin film formation, developed micro-/nano-patterns are easily extracted in water in the form of micro- and nanoparticle mixture of size range 50nm to 3.0µm. This simplified method is also used to effectively encapsulate a dye molecule, rhodamine B inside the PLGA micro-/nanoparticles. The developed dye-encapsulated nanoparticles, PLGA-rhodamine are separated from the mixture and tested for in-vitro delivery application of external molecules inside human lung cancer cells. For the first time, the use of thin film dewetting technique is reported as a potential route for the synthesis of polymeric micro-/nanoparticles and effective encapsulation of external species therein.

Polymeric-Patterned Surface for Biomedical Applications

Polymeric-patterned surfaces are finding significant importance in various biomedical applications such as screening and diagnostic assays, tissue engineering, biosensors, and in the study of fundamental cell biology. A wide variety of methods, involving photolithography, inkjet printing, soft lithography, and dip-pen lithography, have emerged for protein or polymer patterning on various substrates. For directional immobilization or adsorption of protein, surface requires pre-defined regions to which protein molecules can be immobilized. The most common techniques to introduce defined protein immobilization are soft lithography and photolithography. However, these techniques have some associated limitations. In soft lithography, stamps with well-defined structures are required, and the migration of ink during and after printing needs to be well controlled. In photolithography, a polymeric photoresist and a mask are needed which require expensive setup to fabricate. Therefore, facile and economic techniques are worth exploring. The dewetting of a thin polymeric film is a spontaneous and self-organized process that forms an array of microscale and nanoscale droplets on a substrate. This is a facile approach of patterning polymer on glass substrate providing a reliable surface for specific, dense, and uniform immobilization of desired molecules to pre-designed patterns. Since antibody orientation is very important in antibody-based surface capture assays, patterned polymer surfaces are of great importance with respect to an increasing number of biosensor applications. Apart from protein patterning, such polymeric-patterned surface can be effectively used in specific type of cell isolation and detection. Indeed, it is found that circulating tumor cells (CTCs) are easily isolated using such patterned structures either on a flat plate or inside a microfluidic environment.

Ethylenediamine assisted functionalization of self-organized poly (d, l-lactide-co-glycolide) patterned surface to enhance cancer cell isolation

Protein functionalized micro-scale patterned structures are developed using a biocompatible polymer PLGA (poly (d, l-lactide-co-glycolide)) via thin film dewetting and by step-wise chemical conjugations with EDA (ethylenediamine) and anti-EpCAM (Epithelial Cell Adhesion Molecule) antibodies to target the epithelial cell adhesion molecules of cancer cells. The effectiveness of such protein functionalized patterned surface is checked through cell isolation process using blood samples spiked with different cancer cells such as MCF-7, A549, MDA-MB-231. An efficient capture yield of 92% is obtained with MCF-7 cells over a two hour incubation time. The study demonstrates the effects of cell concentration and incubation time on the binding of cancer cells to the modified patterned surfaces. For the first time, a simple and inexpensive method is reported to fabricate functionalized PLGA patterned surface for an efficient isolation of cancer cells from diluted blood samples. The method shows the potential to be used as an effective platform for the development of an improved circulating tumor cell (CTC) isolation device from the clinical blood sample.