Abraham D. Stroock

Components for integrated poly(dimethylsiloxane) microfluidic systems.

This review describes the design and fabrication of microfluidic systems in poly(dimethylsiloxane) (PDMS). PDMS is a soft polymer with attractive physical and chemical properties: elasticity, optical transparency, flexible surface chemistry, low permeability to water, and low electrical conductivity. Soft lithography makes fabrication of microfluidic systems in PDMS particularly easy. Integration of components, and interfacing of devices with the user, is also convenient and simpler in PDMS than in systems made in hard materials. Fabrication of both single and multilayer microfluidic systems is straightforward in PDMS. Several components are described in detail: a passive chaotic mixer, pneumatically actuated switches and valves, a magnetic filter, functional membranes, and optical components.

FLEXIBLE METHODS FOR MICROFLUIDICS

Devices for handling nanoliter quantities of fluids are creating new fabrication challenges and finding new applications in biology, chemistry, and materials science.

Experimental and theoretical scaling laws for transverse diffusive broadening in two-phase laminar flows in microchannels

This letter quantifies both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels at high Peclet numbers. Confocal fluorescent microscopy was used to visualize a fluorescent product formed by reaction between chemical species carried separately by the two solutions. At steady state, the width of the reaction–diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the average velocity of the flow, instead of the more familiar one-half power scaling which was measured in the middle of the channel. A quantitative description of reaction–diffusion processes near the walls of the channel, such as described in this letter, is required for the rational use of laminar flows for performing spatially resolved surface chemistry and biology inside microchannels and for understanding three-dimensional features of mass transport in shearing flows near surfaces.

Pumping based on transverse electrokinetic effects

This work presents a strategy for microfluidic pumping based on transverse electro-osmotic flow in a channel with topographical features on one wall. In this channel, flow along the long axis is generated by an electric field applied across the channel. The pump operates at low (5–10 V) voltage and achieves pumping speeds up to ∼100u2009μm/s in submillimeter channels. The pump is straightforward to fabricate, contains no moving parts, and provides local control of the direction and strength of pumping. The performance of the pump scales favorably with decreasing size of a microchannel.

Competition of intrinsic and topographically imposed patterns in Benard--Marangoni convection

The structure of Benard-Marangoni convection cells can be controlled by periodic topographic patterns on the heated surface that generates the convection. When the periodicity of the topographic pattern matches the intrinsic periodicity of the convection cells, a convective pattern is formed that is 1:1 commensurate with the topographic pattern. Arrays of hexagonal, square, and triangular convection cells were generated over the appropriately designed topographic patterns, and visualized by infrared imaging. For imposed patterns with periodicity in two dimensions, as the ratio of the intrinsic and perturbing length scales changes, the pattern of the convection cells shows sharp transitions between different patterns commensurate with the imposed pattern. For imposed patterns with periodicity in one dimension (i.e., lines) the convection cells use the unconstrained dimension to adapt continuously to the external perturbation. Topographically controlled convection cells can be used to assemble microscopic particles into externally switchable regular lattices

Design Analysis and 3D Measurement of Diffusive Broadening in a Y-mixer

Diffusive broadening of a low molecular weight species in pressure driven flow is studied using both experiment and numerical analysis. Confocal microscopy allows experimental visualization of the three dimensional nature of the diffusion. Numerical results support the experimental results, and are used to provide insight into design questions about devices involving diffusive mixing.

Soft Lithography and Microfluidics

Publisher Summary This chapter describes the application of soft lithography to create and control interface between synthetic materials and biological systems. Soft lithography is a set of techniques that includes first, methods of fabricating microstructures in polymers, especially elastomers, second, uses of these methods in combination with organic surface chemistry to generate micron-scale patterns on synthetic surfaces, and third, uses of microfluidic systems to pattern the composition of the fluid medium adjacent to a surface. These techniques allow the immobilization of biomolecules and cells at surfaces with micron-scale resolution, and for the control of the subsequent interaction of these species with liquid media. These techniques are compatible both with optical and electronic materials and with biological systems. Finally, this chapter reviews the use of soft lithography to fabricate microfluidic systems, and to position and manipulate living cells on surfaces.

Patterning Flows Using Grooved Surfaces: Application to Microfluidics

Pressure-driven flows over grooved surfaces are shown to develop transverse components. This induces helical recirculation in channels or capillaries with grooved walls, and more generally permits the design of 3D flows with simple patterns of grooved regions. This strategy is easy to implement with standard methods of microfabrication, and permits for example the realization of a low Reynolds number chaotic mixer.