Absolute 3D reconstruction of thin films topography in microfluidic channels by interference reflection microscopy.

The travel of droplets, bubbles, vesicles, capsules, living cells or small organisms in microchannels is a hallmark in microfluidics applications. A full description of the dynamics of such objects requires a quantitative understanding of the complex hydrodynamic and interfacial interactions between objects and channel walls. In this paper, we present an interferometric method that allows absolute topographic reconstruction of the interspace between an object and channel walls for objects confined in microfluidic channels. Wide field microscopic imaging in reflection interference contrast mode (RICM) is directly performed at the bottom wall of microfluidic chips. Importantly, we show that the reflections at both the lower and upper surface of the microchannel have to be considered in the quantitative analysis of the optical signal. More precisely, the contribution of the reflection at the upper surface is weighted depending on the light coherence length and channel height. Using several wavelengths and illumination apertures, our method allows reconstructing the topography of thin films on channel walls in a range of 0-500 nm, with a precision as accurate as 2 nm for the thinnest films. A complete description of the protocol is exemplified for oil in water droplets travelling in channels of height 10-400 μm at a speed up to 5 mm s(-1).