Measurement of Low Contact Angle Droplet Topography Using Optical Interferometry

Abstract

Production and manipulation of microdroplets is an active are of research. The demand for finding new ways to produce microdroplets has resulted in a challenge to characterize these droplets. The problem becomes especially difficult for microdroplets which are placed on a transparent hydrophilic substrate. Ink-jet printing, Digital microfluidics, DNA synthesis are some of the applications which need topography measurements of low contact angle microdroplets. One application which is of interest is the sample preparation for Cryo Electron Microscopy (cryo-EM). A recent development in the search for efficient sample preparation for cryo-EM is to use hollow microcantilevers (HMC). HMCs can isolate even a single sub-cellular component and help prepare samples confined in a femtoliter droplet. These droplets are dispensed on a hydrophilic Electron Microscopy grid (EM-grid). By controlling the thickness of the water layer on the EM-grid through evaporation, it is possible to make the HMC technology more reliable by reducing sample wastage. However, such a control loop is absent. A real-time topography measurement of such droplets can serve as a control signal which can be used as feedback for a control loop. The objective of this project is to investigate the feasibility of optical methods to measure the topography information of a low contact angle microdroplet. The scope of this project is limited to develop a tool for a droplet which is supported by a glass slide (droplet-on-glass). In a subsequent study, the tool will be tested for droplets supported on grid (droplet-on-grid). An optical interferometry setup is proposed as a solution. A Mach-Zehnder interferometer is built to obtain the experimental fringes. A Single Frame Fourier Transform technique was used to analyze the data and obtain the results. Droplets of glycerol were dispensed on a glow discharged glass slide. Topography of the droplet was obtained until complete evaporation and a detection limit of 165nm was achieved. The accuracy of the method was found by comparing the results obtained with a Bruker White Light Interferometer (Bruker-WLI) in Phase Shifting Interferometry mode. An accuracy of 23\% was observed. The proposed setup had a repeatability of 14.7nm. To measure the reproducibility of the setup, a 3d printed structure which had the same size and shape as that of a droplet was used. The reproducibility of the setup was found to be 19.8nm over three days. Simulations were performed to analyze the effect of filter shape, filter width and the carrier frequency. Based on these findings, steps to measure a droplet-on-grid system with the proposed setup is explained. Further, to test the capabilities of the instrument the evaporation of a large water droplet on a EM-grid was observed using the proposed setup. It was found that the motion of fringe pattern as the droplet evaporates could give a good indication to control the evaporation time of even conventional machines, which do not employ hollow microcantilevers to dispense small droplets. However, final validation of the proposed setup with the cryo-EM is yet to be performed due to time constraints. As a recommendation for future work, new ways of dispensing samples on the EM-grid are explored. The necessary steps required to validate the setup with a cryo-EM are explained. Further, some ways in which the analysis time could be reduced are explored. This will be helpful in developing a software necessary to implement a real-time solution.