arXiv subjects

Aloke Kumar

Publications and source records attributed to Aloke Kumar.

33 records · Page 2Linked to original sources

arXiv2022

Elasticity can affect droplet coalescence

Recent investigations on the coalescence of polymeric droplets on a solid substrate have reported strong disagreements; the heart of the issue is whether coalescence of polymeric drops is similar to that of Newtonian fluid and is independent of molecular relaxation, or whether the role of entanglement of polymeric chains leads to a transition kinetics different from that of Newtonian fluid. Via this report, we resolve the disagreements through a discussion on the effects of merging method on the dominant forces governing the coalescence process, i.e., inertia, dissipation, and relaxation. Our study unveils that the coalescence dynamics of polymeric drops is not universal and in fact, it is contingent of the method by which the coalescence is triggered. Additionally, we demonstrate the spatial features of the bridge at different time instants by a similarity analysis. We also theoretically obtain a universal bridge profile by employing the similarity parameter in a modified thin film lubrication equation for polymeric fluids.

physics.flu-dyn↗

arXiv2022

Rheocoalescence: Relaxation time through coalescence of droplets

Dynamics of the pendant drop coalescing with a sessile drop to form a single daughter droplet is known to form a bridge. The bridge evolution begins with a point contact between the two drops leading to a liquid neck of size comparable to the diameter of the drops. To probe this phenomenon in polymeric fluids, we quantify the neck radius growth during coalescence using high speed imaging. In the current study, we unveil the existence of three regimes on basis of concentration ratio $c/c^*$ namely, inertio-elastic $c/c^* 20$. Our results suggest that the neck radius growth with time (t) obeys a power-law behaviour $t^b$, such that the coefficient $b$ has a steady value in inertio-elastic and viscoelastic regimes, with a monotonic decrease in elasticity dominated regime. Based on this dependence of $b$ on concentration ratios, we propose a new measurement technique Rheocoalescence to obtain the relaxation time of the these fluids. We also show a deviation from universality proposed in literature for the elasticity dominated regime.

physics.flu-dyn↗

arXiv2021

Coalescence of polymeric sessile drops on a partially wettable substrate

Coalescence of sessile polymeric fluid drops on a partially wettable substrate exhibits a transition from inertial to viscoelastic regime at concentration ratio $c/c^* \sim 1$. Our findings unveil that the temporal evolution of the growing bridge height follows a power-law behaviour $t^b$, such that the coefficient $b$ continuously decreases from 2/3 in the inertial regime ($c/c^*<1$) to an asymptotic value of 1/2 in the visco-elastic regime ($c/c^*>1$). To account for fluid elasticity and characteristic time-scale in the viscoelastic regime, a modified thin film equation under lubrication approximation has been proposed using the linear Phan-Thien- Tanner constitutive equation. The temporal evolution of the droplet has been evaluated by solving the modified one-dimensional thin-film equation using a marching explicit scheme. The initial droplet shapes are obtained by re-sorting to energy minimization. A good agreement between numerical and experimental results is obtained.

physics.flu-dyn↗

arXiv2021

Contact line pinning and de-pinning can modulate the rod-climbing effect

Our experiments on the rod-climbing effect with an oil-coated rod revealed two key differences in the rod-climbing phenomena compared to a bare rod. On one hand, an enhancement in magnitude of climbing height for any particular value of rod rotation speed and second, a decrease in threshold rod rotation speed required for the appearance of the rod-climbing effect were observed. Observed phenomena is explained by considering the contact line behavior at the rod-fluid interface. Transient evolution of meniscus at the rod-fluid interface revealed that the three-phase contact line was pinned for a bare rod and de-pinned for an oil-coated rod. We modelled the subject fluid as a Giesekus fluid to predict the climbing height. The differences in the contact line behaviour were incorporated via the contact angle at the rod-fluid interface as a boundary condition. An agreement was found between the observed and predicted climbing height establishing that contact line behaviour may modulate rod-climbing effect.

physics.flu-dyn↗

arXiv2020

Estimating Hidden Asymptomatics, Herd Immunity Threshold and Lockdown Effects using a COVID-19 Specific Model

A quantitative COVID-19 model that incorporates hidden asymptomatic patients is developed, and an analytic solution in parametric form is given. The model incorporates the impact of lockdown and resulting spatial migration of population due to announcement of lockdown. A method is presented for estimating the model parameters from real-world data. It is shown that increase of infections slows down and herd immunity is achieved when symptomatic patients are 4-6\% of the population for the European countries we studied, when the total infected fraction is between 50-56 \%. Finally, a method for estimating the number of asymptomatic patients, who have been the key hidden link in the spread of the infections, is presented.

q-bio.PE↗

arXiv2019

Universality in coalescence of polymeric fluid drops

Coalescence of liquid droplets involves an interplay between capillary forces, viscous forces and inertial forces. Here, we unveil a universal temporal evolution of the neck radius during the coalescence of two polymeric drops. Through high speed imaging we demonstrate that drops of polyacrylamide (PAM), poly-vinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG) and xanthan gum (XG) depict a universal behavior $R/\sqrt(ν_0λ) \sim (t/λ)^{0.36}(C/C^{*})^{-0.83}$ over a dilute, semi-dilute and non-dilute range of concentrations. A linear Phan-Thein-Tanner viscoelastic model captures the temporal aspect of universality.

physics.flu-dyn↗

arXiv2017

Abiotic streamers in a microfluidic system

In this work, we report the phenomenon of formation of particle aggregates in the form of thin slender strings when a polyacrylamide (PAM) solution, laden with polystyrene (PS) particles is introduced into a microfluidic device containing an array of micropillars. PAM and dilute solution of PS beads is introduced into the microfluidic channel through two separate inlets and localized particle aggregation is found to occur under certain conditions. The particle aggregates initially have a string-like morphology that remain tethered at their ends to the micropillar walls, while the rest of the structure remains suspended in the fluid medium. It is this morphology that inspired us to name these structures streamers. The flow regime under which streamer formation is observed is quantified using through a phase diagram. We discuss the streamer formation time-scales and also show that streamer formation is likely the result of flocculation of the PS beads. These streamers can serve as excellent model systems to study a biological phenomenon by the same name.

physics.flu-dyn↗

arXiv2015

Mesoscale Elucidation of Biofilm Shear Behavior

Formation of bacterial colonies as biofilm on the surface/interface of various objects has the potential to impact not only human health and disease but also energy and environmental considerations. Biofilms can be regarded as soft materials, and comprehension of their shear response to external forces is a key element to the fundamental understanding. A mesoscale model has been presented in this article based on digitization of a biofilm microstructure. Its response under externally applied shear load is analyzed. Strain stiffening type behavior is readily observed under high strain loads due to the unfolding of chains within soft polymeric substrate. Sustained shear loading of the biofilm network results in strain localization along the diagonal direction. Rupture of the soft polymeric matrix can potentially reduce the intercellular interaction between the bacterial cells. Evolution of stiffness within the biofilm network under shear reveals two regions: a) initial increase in stiffness due to strain stiffening of polymer matrix, and b) eventual reduction in stiffness because of tear in polymeric substrate.

physics.flu-dyn↗

arXiv2015

Bacterial floc mediated rapid streamer formation in creeping flows

One of the central puzzles concerning the interaction of low Reynolds number (Re<<1) fluid transport with bacterial biomass is the formation of filamentous structures called streamers. In this manuscript, we report our discovery of a new kind of low Re bacterial streamers, which appear from pre-formed bacterial flocs. In sharp contrast to the biofilm-mediated streamers, these streamers form over extremely small timescales (less than a second). Our experiments, carried out in a microchannel with micropillars rely on fluorescence microscopy techniques to illustrate that floc-mediated streamers form when a freely-moving floc adheres to the micropillar wall and gets rapidly sheared by the background flow. We also show that at their inception the deformation of the flocs is dominated by recoverable large strains indicating significant elasticity. These strains subsequently increase tremendously to produce filamentous streamers. Interestingly, we find that these fully formed streamers are not static structures and show viscous response at time scales larger than their formation time scales. Finally we show that such novel streamer formation can lead to rapid clogging of microfluidic devices.

physics.flu-dyn↗

arXiv2015

Deformation of an elastic body in low Reynolds number transport: Relevance to biofilm deformation and streamer formation

In this paper, we obtain analytical results for shear stress distributions inside an elastic body placed in a low Reynolds number transport. The problem definition is inspired by a recent experimental study (Valiei et al., Lab Chip, 2012, 12, 5133-5137) that reports the flow-triggered deformation of bacterial biofilms, formed on cylindrical rigid microposts, into long filamentous structures known as streamers. In our analysis, we consider an elastic body of finite thickness (forming a rim) placed over a rigid cylinder, i.e., we mimic the biofilm structure in the experiment. We consider Oseen flow solution to describe the low Reynolds transport past this cylindrical elastic structure. The stress and strain distributions inside the elastic structure are found to be functions of position, Poisson ratio, initial thickness of the elastic rim and the ratio of the flow-driven shear stress to the shear modulus of the elastic body. More importantly, these analyses, which can be deemed as one of the first formal analyses to understand the fluid-structure-interaction issues associated with the biofilm streamer formation, help us interpret several qualitative aspects associated with the streamer formation reported in different experiments.

physics.flu-dyn↗

arXiv2014

Type-IV Pilus Deformation Can Explain Retraction Behavior

Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is lowered significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map

physics.bio-ph↗

arXiv2013

Formation and post-formation dynamics of bacterial biofilm streamers as highly viscous liquid jets

It has been recently reported that in presence of low Reynolds number (Re<<1) transport, preformed bacterial biofilms, several hours after their formation, may degenerate in form of filamentous structures, known as streamers. In this letter, we explain that such streamers form as the highly viscous liquid states of the intrinsically viscoelastic biofilms. Such "viscous liquid" state can be hypothesized by noting that the time of appearance of the streamers is substantially larger than the viscoelastic relaxation time scale of the biofilms, and this appearance is explained by the inability of a viscous liquid to withstand an external shear. Further, by identifying the post formation dynamics of the streamers as that of a viscous liquid jet in a surrounding flow field, we can interpret several unexplained issues associated with the post-formation dynamics of streamers, such as the clogging of the flow passage or the exponential time growth of streamer dimensions.

physics.flu-dyn↗

arXiv2009

Light Enabled Digital Microfluidics:A Technology Leading to a Programmable Lab on a Chip

This fluid dynamics video showcases how optically induced electrowetting can be used to manipulate liquid droplets in open space and particulate phases inside the droplet. A photoconductive layer is added to a conventional electrowetting-on-dielectric (EWOD) structure to generate light enabled virtual electrodes, hence resulting in an eletrowetting action. Coplanar electrodes deployed alternately on a substrate enable open droplet manipulations differentiating from a sandwiched configuration. An integration with an optoelectric method shows dynamic and rapid particle handling by strong micro fluidic vortices in conjunction with other electrokinetic forces inside a droplet. The droplet manipulations are realized with visible illumination and powered at 150 volts peat-to-peak with a low frequency (100 Hz-800 Hz). The particle concentration is achieved on the surface of the same chip but illuminated with a near-infrared (1064 nm) light source and biased with a high frequency (24 kHz) AC signal.

physics.flu-dyn↗

arXiv2008

Optically induced electrokinetic patterning and manipulation of particles

This fluid dynamics video showcases how optically induced electrokinetic forces can be used to drive three-dimensional micro-vortices. The strong microfluidic vortices are used constructively in conjunction with other electrokinetic forces to dynamically and rapidly aggregate particle groups. Particle manipulation is achieved on the surface of a parallel-plate gold/indium tin oxide (ITO) electrode that is illuminated with near-infrared (1064 nm) optical patterns and biased with a low frequency ($<$ 100 kHz) alternating current (AC) signal. The fluid dynamics video shows how electrokinetically driven flows in the microdomain can be used for non-invasive particle manipulation.

physics.flu-dyn↗

arXiv2004

Dynamics of a piecewise smooth map with singularity

Experiments observing the liquid surface in a vertically oscillating container have indicated that modeling the dynamics of such systems require maps that admit states at infinity. In this paper we investigate the bifurcations in such a map. We show that though such maps in general fall in the category of piecewise smooth maps, the mechanisms of bifurcations are quite different from those in other piecewise smooth maps. We obtain the conditions of occurrence of infinite states, and show that periodic orbits containing such states are superstable. We observe period-adding cascade in this system, and obtain the scaling law of the successive periodic windows.

nlin.CD↗