Microfluidics: Fluid physics at the nanoliter scale

Abstract

Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes ͑nanoliters͒ of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Péclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world. CONTENTS

An optical toolbox for total control of droplet microfluidics

“…Droplets are natural candidates for use as microreactors, since they transport fluid with no dispersion and may be formed and manipulated using microfluidic techniques [1,2,3,4]. Indeed, a drop may be formed with a known composition and volume [5,6,7] and transported by an inert fluid without loss of the solute species and without cross-contamination [8].…”

One‐Flow, Multistep Synthesis of Nucleosides by Brønsted Acid‐Catalyzed Glycosylation

“…These anomalous results could be overcome by simply employing a reactor with a smaller inner diameter (0.5 mm) which promotes a more uniform laminar flow profile and hence increased accuracy of the residence times (see Supporting Information for details). [13] In the case of cytosine, guanine, and adenine (2g-i), residual bis(trimethylsilyl)acetamide (BSA) from the silylation step had no negative influence in the glycosylation reaction.…”

Direct Measurements of Colloidal Solvophoresis under Imposed Solvent and Solute Gradients

“…18 Recent advances in microfluidics, however, have enabled the direct visualization of diffusiophoretic migration, since the small dimensions of microfluidic devices are effective in suppressing buoyancy-driven instabilities. 19 Colloidal streams have been focused and spread using coflowing streams of varying salinity, 20,21 and agarose-based microfluidic devices have been used to drive the diffusiophoretic motion of colloids and DNA under imposed salinity gradients. 22,23 However, these methods have not yet been used to directly measure concentrationdependent diffusiophoretic mobilities, nor to measure the diffusiophoretic migration under nonelectrolyte gradients, which had been predicted theoretically 4,24,25 and verified experimentally.…”