Exploring the precision of backlight optical imaging in microfluidics close to the diffraction limit

We evaluate the precision of backlight optical imaging to locate the position of a free surface close to the diffraction limit. For this purpose, we acquired and processed images of liquid films at equilibrium deposited on a capillary 10 mu m in diameter. The detected contours were compared with the solution of the exact Young-Laplace equation. Deviations of about 10 nm were obtained, which constitutes the first quantitative validation of backlight optical imaging for fluid shapes a few microns in size. The local mean curvature measured in the experiments agreed remarkably with the theoretical values. We illustrate the applicability of this technique by analysing five interfacial phenomena: (i) the flow in the tip of a capillary tapering meniscus formed by flow focusing, (ii) the extensional deformation of viscous micro-jets by the action of a co-flowing gas stream, (iii) the growth of small-amplitude capillary waves on a micro-jet free surface, (iv) the flow in the tip of a capillary tapering meniscus produced by electrospray, and (v) the characterization of glass micro-nozzles. (C) 2011 Elsevier Ltd. All rights reserved.