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 μm 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.

► We detect liquid free surfaces close to the diffraction limit by optical imaging. ► We analyze images of liquid films over a capillary of 10 microns in diameter. ► Deviations of about 10 nm from the Young-Laplace equation solution are obtained. ► We successfully apply this technique to five interfacial phenomena.