Development and optimization of multifunctional devices for fluidic manipulation of films, drops, and bubbles require detailed understanding of interfacial phenomena and microhydrodynamic flows. Systems are distinguished by a large surface to volume ratio and flow at small Reynolds, capillary, and Bond numbers are strongly influenced by boundary effects and therefore amenable to control by a variety of surface treatments and surface forces. We review the principles underlying common techniques for actuation of droplets and films on homogeneous, chemically patterned, and topologically textured surfaces by modulation of normal or shear stresses.

We investigate the behavior of the slip length in Newtonian liquids subject to planar shear bounded by substrates with mixed boundary conditions. The upper wall, consisting of a homogenous surface of finite or vanishing slip, moves at a constant speed parallel to a lower stationary wall, whose surface is patterned with an array of stripes representing alternating regions of no shear and finite or no slip. Velocity fields and effective slip lengths are computed both from molecular dynamics ͑MD͒ simulations and solution of the Stokes equation for flow configurations either parallel or perpendicular to the stripes. Excellent agreement between the hydrodynamic and MD results is obtained when the normalized width of the slip regions, a / տ O͑10͒, where is the ͑fluid͒ molecular diameter characterizing the Lennard-Jones interaction. In this regime, the effective slip length increases monotonically with a / to a saturation value. For a / Շ O͑10͒ and transverse flow configurations, the nonuniform interaction potential at the lower wall constitutes a rough surface whose molecular scale corrugations strongly reduce the effective slip length below the hydrodynamic results. The translational symmetry for longitudinal flow eliminates the influence of molecular scale roughness; however, the reduced molecular ordering above the wetting regions of finite slip for small values of a / increases the value of the effective slip length far above the hydrodynamic predictions. The strong correlation between the effective slip length and the liquid structure factor representative of the first fluid layer near the patterned wall illustrates the influence of molecular ordering effects on slip in noninertial flows.

Abstract-We have designed a microfluidic device for the actuation of liquid droplets or continuous streams on a solid surface by means of integrated microheater arrays. The microheaters provide control of the surface temperature distribution with high spatial resolution. These temperature gradients locally alter the surface tension along droplets and thin films thus propelling the liquid toward the colder regions. In combination with liquophilic and liquophobic chemical surface patterning, this device can be used as a logistic platform for the parallel and automated routing, mixing and reacting of a multitude of liquid samples, including alkanes, poly(ethylene glycol) and water.[1024]

Open microfluidic devices based on actuation techniques such as electrowetting, dielectrophoresis, or thermocapillary stresses require controlled motion of small liquid droplets on the surface of glass or silicon substrates. In this article we explore the physical mechanisms affecting thermocapillary migration of droplets generated by surface temperature gradients on the supporting substrate. Using a combination of experiment and modeling, we investigate the behavior of the threshold force required for droplet mobilization and the speed after depinning as a function of the droplet size, the applied thermal gradient and the liquid material parameters. The experimental results are well described by a hydrodynamic model based on earlier work by Ford and Nadim. The model describes the steady motion of a two-dimensional droplet driven by thermocapillary stresses including contact angle hysteresis. The results of this study highlight the critical role of chemical or mechanical hysteresis and the need to reduce this retentive force for minimizing power requirements in microfluidic devices.

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...