Shantimoy Kar

Generation of droplets to serpentine threads on a rotating compact-disk platform

We generate stable monodisperse droplets of nano-liter volumes and long serpentine liquid threads in a single, simple “Y”-shaped microchannel mounted on a rotationally actuated lab-on-a-compact-disk platform. Exploitation of Coriolis force offers versatile modus operandi of the present setup, without involving any design complications. Based on the fundamental understanding and subsequent analysis, we present scaling theories consistent with the experimental observations. We also outline specific applications of this technique, in the biological as well as in the physical domain, including digital polymerase chain reaction(PCR), controlled release of medical components, digital counting of colony forming units, hydrogelengineering, optical sensors and scaffolds for living tissues, to name a few.

Formation of Blood Droplets: Influence of the Plasma Proteins

Blood is a complex multiphase fluid exhibiting pronounced shear-thinning and viscoelastic behavior. By studying the formation of blood droplets through simple dripping, we observe blood-drop detachment following a neck formation and subsequent thinning until breakup, similar to that of other liquids. Our experimental findings reveal that it exhibits two distinct modes of neck evolution characteristics; one mode corresponds to incessant collapsing of the liquid neck, whereas the other mode correlates thinning of an extended long thread leading to the breakup. We show that the two modes of neck evolution closely follow the theory of pinch-off for shear-thinning and viscoelastic fluids independent of hematocrit concentration in the range of healthy individuals. Furthermore, we observe that the relaxation time scales are very similar to that of plasma; this explains the key role of plasma proteins to blood rheology. We envision that our results are likely to bear far-reaching implications in understanding the contribution of plasma proteins to the rheology of blood and theory of drop formation of complex non-Newtonian fluids.

Predicting Escherichia coli's chemotactic drift under exponential gradient

Bacterial species are known to show chemotaxis, i.e., the directed motions in the presence of certain chemicals, whereas the motion is random in the absence of those chemicals. The bacteria modulate their run time to induce chemotactic drift towards the attractant chemicals and away from the repellent chemicals. However, the existing theoretical knowledge does not exhibit a proper match with experimental validation, and hence there is a need for developing alternate models and validating experimentally. In this paper a more robust theoretical model is proposed to investigate chemotactic drift of peritrichous Escherichia coli under an exponential nutrient gradient. An exponential gradient is used to understand the steady state behavior of drift because of the logarithmic functionality of the chemosensory receptors. Our theoretical estimations are validated through the experimentation and simulation results. Thus, the developed model successfully delineates the run time, run trajectory, and drift velocity as measured from the experiments.

Diffusive dynamics on paper matrix

Writing with ink on a paper and the rapid diagnostics of diseases using paper cartridge, despite their remarkable diversities from application perspective, both involve the motion of a liquid from a source on a porous hydrophilic substrate. Here we bring out a generalization in the pertinent dynamics by appealing to the concerned ensemble-averaged transport with reference to the underlying molecular picture. Our results reveal that notwithstanding the associated complexities and diversities, the resultant liquid transport characteristics on a paper matrix, in a wide variety of applications, resemble universal diffusive dynamics. Agreement with experimental results from diversified applications is generic and validates our unified theory.