Monitoring the evaporation dynamics of a water droplet inside a microcapillary with nanometre-scale precision

There is a growing interest to the multi-disciplinary field of droplet microfluidics having applications in physics, chemistry, biology, surface science, colloidal liquid dynamics etc. [1, 2]. Usually, optical sensing methods characterize the droplets with the spatial resolution determined by the wavelength of light, i.e., comparable with 1 micron. Alternatively, the approach suggested in Ref. [3] allows to determine the refractive index variation of submicron-thick liquid layers adjacent to the surface of microcapillaries. Microfluidic sensing in Ref. [3] was performed by measurement of the resonant whispering gallery mode (WGM) spectrum. Specifically, WGMs were launched into the microcapillary using the normally attached micron-diameter waist of a biconical fibre taper connected to the light source and optical spectrum analyser (OSA) as illustrated in Fig. 1(a). Evanescent coupling of WGMs to the interior of the microcapillary allowed to detect the refractive index variation of liquid by monitoring the shifts of the WGM resonances.