Pramod Chamarthy

Experimental investigation of steady buoyant-thermocapillary convection near an evaporating meniscus

Micro-particle image velocimetry measurements of the three-dimensional (3D) convection patterns generated near an evaporating meniscus in horizontally oriented capillary tubes are presented. Analysis of the vapor diffusion away from the meniscus reveals a zone of intense heat flux near the solid-liquid-vapor junction that creates a temperature gradient along the meniscus. This results in a surface tension gradient which, coupled with buoyancy effects, causes buoyant-thermocapillary convection in the liquid film. The relative influence of buoyancy and thermocapillarity on the flow was investigated for tube diameters ranging from 75 to 1575μm. A transition from a pure two-dimensional thermocapillary flow to a 3D buoyant-thermocapillary flow is observed with an increase in tube diameter. For the 75μm tube, a symmetrical toroidal vortex is observed near the meniscus. For larger tubes, buoyancy effects become apparent as they dominate the flow field. The high mass fluxes in smaller-diameter tubes drive stronge...

Comparison of Experiments and Simulation of Joule Heating in ac Electrokinetic Chips

ac electrokinetic manipulations of particles and fluids are important techniques in the development of lab-on-a-chip technologies. Most of these systems involve planar microelectrode geometries, generating high strength electric fields. When these fields are applied to a dielectric medium, Joule heating occurs. Understanding electrothermal heating and monitoring the temperature in these environments are critical for temperature-sensitive investigations including biological applications. Additionally, significant changes in fluid temperature when subjected to an electric field will induce electrohydrodynamic flows, potentially disrupting the intended microfluidic profile. This work investigates heat generated from the interaction of ac electric fields and water at various electrical conductivities (from 0.92 mS/m to 390 mS/m). The electrode geometry is an indium tin oxide (ITO) electrode strip 20 μm wide and a grounded, planar ITO substrate separated by a 50 μm spacer with microfluidic features. Laser-induced fluorescence is used to measure the experimental changes in temperature. A normalization procedure that requires a single temperature-sensitive dye, Rhodamine B (RhB), is used to reduce uncertainty. The experimental electrothermal results are compared with theory and computer simulations.

Experimental Investigation of Thermocapillary Convection Near an Evaporating Meniscus

Experimental visualizations of the 3D convection patterns generated near an evaporating meniscus in a capillary tube are presented. Epi-fluorescent micro-particle image velocimetry (μPIV) using a two-cavity frequency doubled ND-YAG laser as the illumination source is employed to map the small-scale spatial flow fields near the meniscus. Methanol seeded with 0.5 μm polystyrene fluorescent particles is used as the experimental fluid. These fluorescent particles absorb light from the laser beam (λabs~542 nm, green) and emit a longer wavelength light ((λem~612 nm, red). Images obtained at a specified time delay (~20 ms) were interrogated to obtain vector flow fields.Copyright

Microscale Laser-Induced Fluorescence Method for Non-Intrusive Temperature Measurement

Ratiometric Laser Induced Fluorescence (LIF) Thermometry is applied for temperature measurements in a ‘T’ junction, using microscale visualization methods. Rhodamine B (RhB) and Rhodamine 110 (Rh110) are used as the temperature-dependent and temperature-independent dye, respectively. The temperature responses of the two dyes were carefully measured for different concentrations. A novel normalization procedure for the calibration curve is proposed to render the technique system-independent. The mixing plane between a hot and a cold fluid stream for three different temperatures and three different flow rate ratios is visualized using 4X and 10X magnifications.© 2007 ASME

Simultaneous Measurement of Temperature and Velocity Using µPIV

In μPIV, for a uniform velocity field the displacement of the cross-correlation function gives the velocity of the fluid and the broadening of the peak-width represents the amount of Brownian motion present. In the presence of a linear or a parabolic shear, the shape of the cross-correlation function would have both the Brownian motion information as well as the velocity distribution information. In the present work, the broadening of the cross-correlation function caused by the velocity gradient was subtracted from the total peak broadening in order to isolate the Brownian motion information and thus infer temperature. To the authors knowledge, this technique has not been applied to measure the temperature of a moving fluid. The experiments were conducted in a gravity driven flow through a tube surrounded by a constant temperature water bath.Copyright

Mixing Characteristics in a Serpentine Micro-Channel

Mixing characteristics in a 2D serpentine micro-channel were studied experimentally to understand the role of Dean vortices in enhancing mixing. Mixing plane deformation was visualized at Reynolds numbers ranging from 1 to 200 with the corresponding Dean numbers ranging from 1 to 205. The 2D serpentine micro-channel was found to be a poor mixer for Re 100. This study will have a significant impact on the understanding of transport properties of flows in similar geometries.Copyright