BITS Faculty Publications
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Item Evaporation kinetics of laser modulated pendant nanocolloidal droplet(Begell House, 2023) Harikrishnan, A.R.While a body of literature is there on the sessile evaporation of droplets, literatures dealing with the evaporation characteristics of the complex nanocolloidal systems are scarce. While a few literatures deals with the evaporation kinetics of such colloids the effect of the external optical irradiation in modulating the evaporation kinetics are not talked in literature. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of the hanging nanocolloidal droplets which are free from surface effects so as to capture the physics behind the interfacial mass transport. The current study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of such complex colloidal systems. Evidence of internal circulation was observed with PIV technique in colloidal systems together with volumetric heat generation which can be attributed to be the causes behind the enhanced evaporation rate. Theoretical analysis of the evaporation rate with the classical mass transfer model for droplets falls short in predicting the evaporation rate in colloidal systems. Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found to induce the circulation in such systems.Item Electromagnetic field orientation and dynamics governs advection characteristics within pendent droplets(ARXIV, 2018-07) Harikrishnan, A.R.The article reports the domineering governing role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendant droplets. Literature shows that solutal advection drives circulation cells within salt based droplets. Flow visualization and velocimetry reveals that the direction of the applied field governs the enhancement/reduction in circulation velocity and the directionality of circulation inside the droplet. Further, it is noted that while magnetic fields augment the circulation velocity, the electric field leads to deterioration of the same. The concepts of electro andmagnetohydrodynamics are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The model is found to reveal the roles of and degree of dependence on the governing Hartmann, Stuart, Reynolds and Masuda numbers. The theoretical predictions are observed to be in good agreement with experimental average spatio-temporal velocities. The present findings may have strong implications in microscale electro and/or magnetohydrodynamics.Item The competing effects of high zeta potential and finite ionic size on the thermal behaviour of pseudoplastic flows through confined spaces with hydrophobic surfaces(Begell House, 2021-12) Harikrishnan, A.R.We present Galerkin Finite Element computations of the temperature profile and Nusselt Number associated with the mixed electroosmotic and pressure driven flow of a power law non - Newtonian fluid through a micro/nano channel with velocity slip at the wall. The geometry considered is a parallel plate microchannel and the mathematical model used incorporates the effects of high zeta potential, steric effect, viscous dissipation and Joule heating. Based on the temperature profile and Nusselt Number, we determine the qualitative effect of various hydrodynamic and thermal parameters on the heat transfer performance of the flow and in particular, the influence of the zeta potential and the steric factor. We observe that the zeta potential and steric factor have competing effects that are most prominent near the wall of the microchannel. It is also observed that velocity slip at the wall can enhance the overall convective heat transfer in the fluid. The results of this study offer insight into the thermal design of micro/nano-fluidic systems.Item Droplet Collision and Nucleation Hydrodynamics on Superhydrophobic Cylindrical Surfaces(Springer, 2023-04) Harikrishnan, A.R.Water drop impact onto hydrophobic cylindrical surfaces with four different curvature ratio were experimentally investigated. At lower Weber number impact droplet asymmetrically bounces from all curvature cases with increase in Weber number droplet starts splitting/splashing. On higher striking velocity, the stretched lamella shatters into several small droplets. The high velocity impact droplets ruptures rapidly by formation of nucleation holes on the film as a result of small scale roughness on contact surface. The small scale roughness on test surface causes hole nucleation/film rupturing and reduces the contact time. As the impinging velocity reaches the maximum of our experimental study, the contact time was observed to be even less that the capillary time (tc < τ0). Due to complete shattering of water drop, the retraction time is absent in these cases and results in reduced contact time. It was found that the number of nucleations is in proportion with velocity of impact and contact area on striking.Item Mixed Pressure - Driven and Electrohydromagnetic flows of Power Law Fluids through narrow confinements at high zeta potentials(Springer, 2023-04) Harikrishnan, A.R.We investigate the flow a non – Newtonian (power law fluid) through a parallel plate microchannel under the combined action of a pressure gradient, axial electric field and perpendicular magnetic field (electromagnetohydrodynamic EMHD flow). We examine the interplay of both favourable adverse pressure gradients, high zeta potentials and the superimposed magnetic field. It is observed that the perpendicular magnetic field suppresses the velocity profile is aided by an adverse pressure gradient. The magnetic field competes against both favourable pressure gradient and high zeta potential. At low magnetic field strengths, a significant flow reversal is observed at the channel centre and the strength of the magnetic field increases, the extent to which the flow is reversed reduces. The effect of fluid viscosity is also investigated by varying the dimensionless flow consistency index of the power law fluid. We find that EMHD flow is very sensitive to the viscous properties of the fluid and that even small variations in the flow consistency index can result in a large change in the flow speed.Item Acknowledgment to the Reviewers of Fluids in 2022(MDPI, 2023) Harikrishnan, A.R.High-quality academic publishing is built on rigorous peer review. Fluids was able to uphold its high standards for published papers due to the outstanding efforts of our reviewers. Thanks to the efforts of our reviewers in 2022, the median time to first decision was 42 days and the median time to publication was 17 days. Regardless of whether the articles they examined were ultimately published, the editors would like to express their appreciation and thank the following reviewers for the time and dedication that they have shown Fluids:Item Scaling analysis for azimuthal spreading and contact time of droplet impacting on superhydrophobic cylindrical surfaces(AIP, 2023-09) Harikrishnan, A.R.Drop impact on superhydrophobic surfaces has gained great attention because of its physics and application in water repellency, drag reduction, and anti-icing. Spreading lengths and the contact time are the crucial parameters determining the extend of drop–surface interaction and effective heat transfer between the two and are, hence, trivial to many engineering applications. Post-collisional dynamics over cylindrical geometries are quite different from that of the flat surfaces due to the asymmetry in spreading and retraction dynamics. The dynamics are mainly governed by the impact Weber number and curvature ratio of impacting surface to drop. The spreading dynamics in axial direction is found to be fairly predicted by the governing laws coined for flat surfaces. However, the spreading dynamics in the azimuthal direction is quite complex. Herein, we propose a simple scaling analysis for the spreading dynamics in the azimuthal direction as well as for the contact time of the impacting drop with the surface. A modified capillary length is proposed accounting the curvature effect of the substrate by incorporating a centrifugal component of acceleration for the expanding lamella over the curved surface. With the proposed modified capillary length, a universal scaling relationship for azimuthal spreading length and contact time is developed. The proposed scaling laws are found to be in good agreement with the experimental results from the present study as well as with the existing literature for a wide range of Weber numbers and surface curvature.Item Internal advection dynamics in sessile droplets depend on the curvature of superhydrophobic surfaces(ARXIV, 2018-06) Harikrishnan, A.R.The article demonstrates that the internal circulation velocity and patterns in sessile droplets on superhydrophobic surfaces is governed by the surface curvature. Particle Image Velocimetry reveals that increasing convexity deteriorates the advection velocity whereas concavity augments it. A scaling model based on the effective curvature modulated change in wettability can predict the phenomenon, but weakly. Potential flow theory is appealed to and the curvatures are approximated as wedges with the rested droplet engulfing them partly. The spatially averaged experimental velocities are found to conform to predictions. The study may have strong implications in thermofluidics transport phenomena at the microscale.Item Interplay of high zeta potential and steric factor on the slip thermofluidics of power law fluids through narrow confinements(Elsevier, 2022-04) Harikrishnan, A.R.Thermal transport in a non-Newtonian fluid flow through a parallel plate microchannel is investigated with an emphasis on the effects of high zeta potential and steric factor. The fluid viscosity is modelled using the power law and the flow is driven by a combination of electric field and pressure gradient in the form of a dimensionless flow actuation coefficient. The effects of joule heating, viscous dissipation and slip length at the wall are considered. A semi-analytical formulation for the temperature profile is developed which is solved numerically using the Galerkin Finite Element method. We find that the Nusselt number increases as the flow behaviour index is increased. Velocity slip is also found to have a positive impact on convective heat transfer. However, as the zeta potential and steric factor increase, the Nusselt number begins to decline as there is an increase in viscous dissipation. We highlight the importance of considering variable electrical conductivity in evaluating the effect of joule heating at high zeta potential and moderate steric factor. In this regime, joule heating dominates advective transport and heat flow is reversed which may be detrimental or even damaging to microfluidic devices. A comprehensive thermal map is developed for a wide range of zeta potentials and steric factors.Item Interplay of substrate inclination and wettability on droplet impact dynamics(ARXIV, 2019-02) Harikrishnan, A.R.Experimental investigations were carried out to elucidate the role of surface wettability and inclination on the post impact dynamics of droplets. Maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number for superhydrophobic surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, incorporated with the modifications for the oblique impact conditions. Energy ratios and elongation factor were also measured for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch off at the contact line and inclination dependent elongation mechanism were observed. Contrary to SH surfaces, hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and normal Weber number respectively. This was due to the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Finally, colloidal solutions of nanoparticles were used to elucidate slip and disjoining pressure on SH and hydrophilic surfaces, respectively. Overall, the article provides a comprehensive picture of post impact dynamics of droplets on inclined surfaces encompassing a broad spectrum of governing parameters like Reynolds number, Weber number, degree of inclination and surface wettability.