Kaustav Chaudhury

Alteration of chaotic advection in blood flow around partial blockage zone: role of hematocrit concentration

Spatial distributions of particles carried by blood exhibit complex filamentary pattern under the combined effects of geometrical irregularities of the blood vessels and pulsating pumping by the heart. This signifies the existence of so called chaotic advection. In the present article, we argue that the understanding of such pathologically triggered chaotic advection is incomplete without giving due consideration to a major constituent of blood: abundant presence of red blood cells quantified by the hematocrit (HCT) concentration. We show that the hematocrit concentration in blood cells can alter the filamentary structures of the spatial distribution of advected particles in an intriguing manner. Our results reveal that there primarily are two major impacts of HCT concentrations towards dictating the chaotic dynamics of blood flow: changing the zone of influence of chaotic mixing and determining the enhancement of residence time of the advected particles away from the wall. This, in turn, may alter the extent of activation of platelets or other reactive biological entities, bearing immense consequence towards dictating the biophysical mechanisms behind possible life-threatening diseases originating in the circulatory system.

Droplet migration characteristics in confined oscillatory microflows.

We analyze the migration characteristics of a droplet in an oscillatory flow field in a parallel plate microconfinement. Using phase field formalism, we capture the dynamical evolution of the droplet over a wide range of the frequency of the imposed oscillation in the flow field, drop size relative to the channel gap, and the capillary number. The latter two factors imply the contribution of droplet deformability, commonly considered in the study of droplet migration under steady shear flow conditions. We show that the imposed oscillation brings an additional time complexity in the droplet movement, realized through temporally varying drop shape, flow direction, and the inertial response of the droplet. As a consequence, we observe a spatially complicated pathway of the droplet along the transverse direction, in sharp contrast to the smooth migration under a similar yet steady shear flow condition. Intuitively, the longitudinal component of the droplet movement is in tandem with the flow continuity and evolves with time at the same frequency as that of the imposed oscillation, although with an amplitude decreasing with the frequency. The time complexity of the transverse component of the movement pattern, however, cannot be rationalized through such intuitive arguments. Towards bringing out the underlying physics, we further endeavor in a reciprocal identity based analysis. Following this approach, we unveil the time complexities of the droplet movement, which appear to be sufficient to rationalize the complex movement patterns observed through the comprehensive simulation studies. These results can be of profound importance in designing droplet based microfluidic systems in an oscillatory flow environment.

Spreading of a droplet over a nonisothermal substrate: multiple scaling regimes

We envisage the spreading behavior of a two-dimensional droplet under a thin-film-based paradigm, under a perfect wetting condition, while the droplet is placed over a nonisothermal substrate. Starting from the onset of thin-film behavior (or equivalently beyond the inertia-dominated initial stage), we identify the existence of mutually contrasting multiple scaling regimes defining the spreading behavior at different time scales. This is attributable to the time-stage-wise upsurge of capillarity or thermocapillarity over the other. In particular, the spreading behavior is characterized by the foot-width (w) evolution with time (t) in a power-law fashion w ∼ t(α), with α being the spreading exponent, defining the rate of spreading. Following pertinent thin-film and subsequent similarity analysis, we identify different asymptotes of α over disparate temporal scales, leading to the characterization of different scaling regimes over the entire spreading event starting from the inception of thin-film behavior. Reported literature data are found to correspond well to the present interpretations and estimations.

Paper-PDMS hybrid microchannel: a platform for rapid fluid-transport and mixing

The functionalities of a paper-PDMS hybrid microchannel, as a potential fluidic transport platform, are presented. The setup consists of a PDMS microchannel with one of its walls covered by paper. In contrast to the available microfluidic platforms, the capillary filling is found to occur at much faster speed in the hybrid channel. Moreover, experimentation using two dye solutions shows mixing enhancement at a significantly faster rate and at a shorter distance in the hybrid channel as compared to the other available counterparts. The paper attachment is found to induce an effective slip during liquid transport, and thereby allows faster transport and capillary filling. The liquid slippage further modifies the shear flow behavior near the wall leading to a slip-enhanced mixing within the hybrid channel. These fundamental understandings correspond to the experimental results quantitatively in terms of corroborating scaling laws. Further mixing enhancement is introduced through spiral, curved and elliptical–spiral geometries of the channel. Apart from the above benefits, the enclosed arrangement protects sensitive reagents from external environment and offers better control over their transport, thus giving a stable mixing and reaction performance inside the channel.

Electrokinetic Maneuvering of Bubble-Driven Inertial Micro-Pumping Systems

The pumping of an aqueous electrolyte by means of an asymmetrically placed thermal resistor and electrodes is investigated in this work. This device has no moving parts and provides a continuous and controllable pulsating flow, which make it a very attractive and viable option for use on lab-on-a-chip devices. The electric field induced modulation provides a higher degree of control on the mass flow rate, by means of which one can achieve on-the-fly mass flow rate control. The pumping action is achieved by means of a high-pressure bubble generated by actuating a thermal resistor which is located asymmetrically between two reservoirs. The ends of the channel are connected to fluidic columns. The combined action of an applied electric field and a faster refilling of the shorter arm after bubble collapse essentially drive a net amount of electrolyte through the system. We study the influence of the geometric parameters like the location of the heater, channel width and the channel length apart from the physiochemical parameters like the Debye length and the applied field strength on the mass flow rate achieved through this device.

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.

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.