Ashis Kumar Sen

Mixing of fluids in microchannels is challenging due to the inherent laminar nature of the fluid flows. This work presents numerical investigations of an active micromixer with an array of lateral electrode pattern along the microchannel. The proposed micromixer is simulated using a numerical model that solves the coupled flow, electric field and diffusion equations with appropriate boundary conditions. The geometry of the micromixer is illustrated and the numerical model is described. The simulation model is validated by comparing the model predictions with experimental results. The performance of the active micromixer is compared with that of an equivalent passive micromixer indicating the advantage. An extensive parametric study is carried out to investigate mixing performance of the proposed micromixer with respect to various geometrical and operational parameters. The layout of the electrodes parallel and staggered electrode arrangements are studied and compared. Effects of electric potential, Reynolds number, Peclet number, Zeta potential, electrode arrangements, gap between electrodes, and number electrode pairs on the mixing performance is studied and discussed. © 2012 Bentham Science Publishers.

Here, we report electrospray performance of multiple emitters (of internal diameter 200 μm) arranged in a linear (inline) array. For a fixed flow rate Q, at higher voltages Q, multi-jet mode is observed, which leads to a rapid increase in the spray current as compared to the single cone-jet case. A theoretical model is presented that predicts (within 10% of experimental data) the divergence of sprays issued from a pair of interacting emitters due to the mutual Columbic interaction of space charges. The variation of onset voltage and spray current with spacing between the emitters is studied and it is found that and. The effect of the flow rate Q, voltage and number of emitters on the spray current is investigated and it is found that Q, and . The present work provides insight regarding the behavior of interacting sprays in an inline configuration and could be significant in the design of multiple emitter systems for electrospray applications.

The development of a biosensor for rapid and quantitative detection of the dengue virus continues to remain a challenge. We report a lab-on-chip device that combines membrane-based blood plasma separation and a localized surface plasmon resonance (LSPR) based biosensor for on-chip detection of dengue NS1 antigen from a few drops of blood. The LSPR effect is realized by irradiating UV-NIR light having a spectral peak at 655 nm onto nanostructures fabricated via thermal annealing of a thin metal film. We study the effect of the resulting metal nanostructures on the LSPR performance in terms of sensitivity and limit of detection, by annealing silver films at temperatures ranging from 100 to 500 °C. The effect of annealing temperature on the nanostructure size and uniformity and the resulting optical characteristics are investigated. Further, the binding between non-targeted blood plasma proteins and NS1-antibody-functionalized nanostructures on the LSPR performance is studied by considering different blocking mechanisms. Using a nanostructure annealed at 200 °C and 2X-phosphate buffer saline with 0.05% Tween-20 as the blocking buffer, from 10 μL of whole blood, the device can detect NS1 antigen at a concentration as low as 0.047 μg mL-1 within 30 min. Finally, we demonstrate the detection of NS1 in the blood samples of dengue-infected patients and validate our results with those obtained from the gold-standard ELISA test.

We demonstrated the detection and isolation of fluorescently tagged cancer cells (HeLa and DU145) from a mixed population of cells and peripheral blood mononuclear cells at a concentration range ͳͲ4-ͳͲ6 at 300 cells per second in a single-cell droplet format. Initially, cells are focused in the single-file stream using sheath fluid before getting encapsulated in the droplets at the droplet junction. The cell encapsulated droplet then analysed and characterised based on different scatter and fluorescent signals at the optical detection zone. The fluorescent signals from the target cells activate a selective electro-coalescence isolation module which uses a very low voltage (60V).

We report a simple, inexpensive, rapid, and one-step method for the fabrication of a stable and biocompatible superhydrophobic and superhemophobic surface. The proposed surface comprises candle soot particles embedded in a mixture of PDMS+n-hexane serving as the base material. The mechanism responsible for the superhydrophobic behavior of the surface is explained, and the surface is characterized based on its morphology and elemental composition, wetting properties, mechanical and chemical stability, and biocompatibility. The effect of %n-hexane in PDMS, the thickness of the PDMS+n-hexane layer (in terms of spin coating speed) and sooting time on the wetting property of the surface is studied. The proposed surface exhibits nanoscale surface asperities (average roughness of 187 nm), chemical compositions of soot particles, very high water and blood repellency along with excellent mechanical and chemical stability and excellent biocompatibility against blood sample and biological cells. The water contact angle and roll-off angle is measured as 160° ± 1° and 2°, respectively, and the blood contact angle is found to be 154° ± 1°, which indicates that the surface is superhydrophobic and superhemophobic. The proposed superhydrophobic and superhemophobic surface offers significantly improved (>40%) cell viability as compared to glass and PDMS surfaces.

Passive droplet manipulation on open surfaces can be achieved by creating a wettability gradient on surfaces, which is essential in the fabrication of low cost biological and biochemical chips. We performed 3D numerical simulations to analyze the droplet motion on a broad range of wettability gradient surfaces. We found that the droplet shape evolves with time to maintain a minimum energy state, and the surface energy of the droplet is identical at a particular non-dimensional time (t∗) for different wettability gradient surfaces. Although the droplet is at various locations at a fixed t∗, the shape of the droplet is found to be identical. The physics behind this interesting phenomenon of identical droplet shape formation is explored. A co-relation for t∗ is proposed to get the dependency of t∗ on various geometrical parameters and fluid properties. Three distinct regimes of the droplet identical shape on different wettability gradient surfaces are shown using a regime plot. Along with the identical droplet shape phenomena, the detailed understanding of the dynamics of the droplet shape evolution on different wettability gradient surfaces gives an insight for better open surface passive manipulation.

There is an urgent need for a cost-effective, precise, and portable device for rapid and in situ measurement of the critical properties of an emulsion. Here, we report the development of such an optofluidic device for the measurement of mean droplet size (dMean) and droplet size distribution (DSD) of a water-in-oil emulsion. We formulated and detected water-in-oil droplets of much smaller dimensions (15μm) compared to the detection of larger droplets or plugs ( 100 μm to 300μm) reported in the literature, employing a cost effective and portable in-house built optical detection system. Use of the device for the measurement of the frequency of droplets from an on-chip droplet generator is demonstrated and validated using microscopy with excellent accuracy (2%). In addition, we provide some insight into the relatively high uncertainty in the collected signal in case of smaller droplets. The droplet size dD is characterized in terms of forward scatter signal and residence time T. We further argue that normalized residence time T of droplets in the detection zone which correlates linearly with droplet size dD is a better parameter to measure droplet size , compared to the forward scatter signal which vFSCM correlates nonlinearly with dD. Finally, the device is used to count the number of droplets of different size to predict dMean and DSD of emulsions. The results were compared with that obtained from traditional microscopy and a very good match (10-13%) was found, in contrast to previously reported non-portable off-chip methods that are 20-44% accurate. Thus, the reported device possesses high potential for accurate measurement of and DSD of emulsions in practical applications.

Early diagnosis of dengue biomarkers by employing a technology that is less labor- and time-intensive and offers higher sensitivity and lower limits of detection would find great significance in the developing world. Here, we report the development of a biosensor that exploits the localized surface plasmon resonance (LSPR) effect of silver nanostructures, created via thermal annealing of thin metal film, to detect dengue NS1 antigen, which appears as early as the onset of infection. The biosensor integrates membrane-based blood-plasma separation to develop lab-on-chip device that facilitates rapid diagnosis (within 30 min) of dengue NS1 antigen from a small volume (10 µL) of whole blood. The refractive index (RI) sensitivity of the LSPR biosensor was verified by using aqueous glycerol (0–100 wt%) which showed that it is sufficiently sensitive to detect 10 −3 change in RI, which is comparable to that observed with protein-protein interaction. The RI sensitivity was utilized to demonstrate protein binding by using bovine serum albumin and detection of antibody-antigen immune reaction by binding human chorionic gonadotropin antigen to immunoglobulin antibody immobilized in our LSPR biosensor. Next, we demonstrated the detection of NS1 in plasma obtained via centrifugation and in plasma separated on-chip. From 10 µL of whole blood spiked with NS1 antigen, our biosensor reliably detects 0.06 µg/mL of NS1, which lies within the clinical limit observed during the first seven days of infection, with a sensitivity of 9 nm/(µg/mL). These results confirm that the proposed LSPR biosensor can potentially be used in point-of-care dengue diagnostics.

The mechanical properties of cells have been considered as biomarkers to indicate the presence of various diseases and changes in cell states. In literature, various conventional techniques have been established for studying cell mechanics such as atomic force microscopy (AFM), optical tweezers (OT), micropipette aspiration (MA), and cell stretching. Traditional techniques are time consuming and produce low throughput which is inadequate for time-sensitive analysis as well as has lesser clinical relevance. Toward this, microfluidic techniques provide attractive and suitable platform for cell phenotyping because of comparatively higher throughput, requirement of small sample volume, integration capability, biocompatibility, fast response, and dimensional match with biological cells. In the last few years, various microfluidic techniques have been developed for studying the mechanics of single cells. In this chapter, we present some of the recently developed microfluidic techniques and explain their benefit over the traditional techniques.

We investigate the elastocapillary flow of Newtonian liquid in rectangular PDMS microchannels having a deformable membrane wall. The inward deflection of the membrane under negative capillary pressure is found to increase the filling speed in horizontal deformable microchannels, and the capillary rise in vertical deformable microchannels. A theoretical model is developed to quantitatively predict these effects, and a non-dimensional parameter J, which represents the ratio of capillary force to the mechanical restoring force, is seen to emerge as an important parameter in quantifying elastocapillary effects. The model predictions show good agreement with experimental data obtained from deformable channels fabricated in PDMS.