Microfluidic actuation by modulation of surface stresses

Abstract

We demonstrate the active manipulation of nanoliter liquid samples on the surface of a glass or silicon substrate by combining chemical surface patterning with electronically addressable microheater arrays. Hydrophilic lanes designate the possible routes for liquid migration while activation of specific heater elements determines the trajectories. The induced temperature fields spatially modulate the liquid surface tension thereby providing electronic control over the direction, timing, and flow rate of continuous streams or discrete drops. Temperature maps can be programed to move, split, trap, and mix ultrasmall volumes without mechanically moving parts and with low operating voltages of 2-3 V. This method of fluidic actuation allows direct accessibility to liquid samples for handling and diagnostic purposes and provides an attractive platform for palm-sized and battery-powered analysis and synthesis. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1537512͔Miniaturized automated systems for liquid routing, mixing, analysis, and synthesis are rapidly expanding diagnostic capabilities in medicine, genomic research, and material science.1 Liquid flow in microchannels can be regulated by pressure gradients, 2 thermocapillary pumping, 3 electrokinetic forces, 4,5 or magnetohydrodynamic pumping.6,7Electrowetting 8,9 and dielectrophoresis 10 have also been used to move droplets on an open surface. These techniques typically require high operating voltage and high electrolyte concentrations. We demonstrate a different method for liquid handling and transport that uses programmable surface temperature distributions, in conjunction with chemical substrate patterning, to provide electronic control over the direction, timing, and flow rate. This method capitalizes on the large surface-to-volume ratio inherent in microscale systems since the gas-liquid and liquid-solid surface energies are modulated to induce and confine flow. It works equally well with polar or nonpolar liquids, requires no mechanically moving parts and operates at very low voltages. The open architecture is best suited to liquids of low volatility. Encapsulation schemes that retain the free liquid-air interfaces can minimize evaporative loss.Local heating of a liquid film at a position x reduces the surface tension ␥(x) to produce a thermocapillary shear stress •n ϭ"␥ϭ(‫ץ/␥ץ‬T)"T that pulls liquid toward regions of cooler surface temperature T. [11][12][13] Since ‫‪T‬ץ/␥ץ‬ is essentially temperature independent for all liquids, the driving force and flow direction are proportional to "T. For a thin flat liquid layer, the average flow speed and flow rate ͑per unit width͒ are given by (x)ϭh(x,t) •n /2(x) and Q(x,t)ϭh(x,t)(x,t), where h(x,t) is the film thickness and (x) the local viscosity. This phenomenon forms the basis of our microfluidic device for actuating continuous streams and discrete droplets.The sample layout 14 for driving continuous streams is shown in Fig. 1͑a͒. Hydrophilic stripes connect pairs of 4.5-mm-wide square reservoir pads; the...

Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation

“…All other regions of the chip surface were chemically treated to repel the liquids tested. 8,10 As shown in Fig. 4(b), the presence of three distinct glycerol droplets of slightly different volume locally increased the capacitive signal along the linear array of electrodes (w = 800 mm).…”

The pumping lid: investigating multi-material 3D printing for equipment-free, programmable generation of positive and negative pressures for microfluidic applications

“…The approaches described in this work address many of the fluid-handling challenges that are faced when working with microfluidic devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] including those involving laminar flow, 22,38 droplets, 21,[39][40][41] and cell culture experiments. [42][43][44] The simplicity of this pumping method overall and the use of the guiding structures make it robust to differences in pushing/pulling force; the user simply places a sample at the inlet and then pushes/pulls the pumping lid to generate the flow.…”

Planar digital nanoliter dispensing system based on thermocapillary actuation

“…Methods within this category include electrowetting, 5,6 dielectrophoresis, 7,8 surface-acoustic waves 9 and thermocapillary propulsion. 10 With each of these techniques, discrete droplets can be transported along a planar substrate by activation of embedded electrodes or microheaters.…”

“…It does not occur in the symmetric splitting of long liquid filaments, where the liquid filament thickness above the resistor decreases monotonically. 48 In geometric terms, detachment of a droplet requires a sufficiently high temperature gradient such that the leading edge of the cell meniscus recedes to the left of the center of R 1 . For the device layouts used in this study, this situation corresponds to a leading edge position x apex # 400 mm, as indicated by the white dashed line in Fig.…”