Simultaneous measurement of interaction force and liquid film thickness between two approaching droplets using integrated optical interferometry and interface probe method.

Comprehensive quantitative measurement of the variation in inter-droplet interaction forces is crucial for a deeper understanding of the fundamental nature of droplet-droplet interactions. Existing methods have not yet achieved the measurement of dynamic interaction processes between droplets, highlighting the necessity of developing a novel measurement approach. An apparatus was designed and custom-developed specifically for investigating the dynamic interaction between aqueous droplets in an organic medium, while also examining the effects of different operational conditions on the film drainage process. This apparatus integrates optical interferometry with the interface probe method, enabling simultaneous and accurate measurement of time-dependent inter-droplet interaction forces and the spatiotemporal evolution of the film thickness. Experimental results demonstrate that the measured liquid film thickness distribution aligns with the tangentially immobile Reynolds lubrication theory, and the measured inter-droplet interaction forces are consistent with the theoretical calculations. The findings indicate that, for films with identical areas, more pronounced initial dimples result in shorter drainage times, while stronger inter-droplet interaction forces lead to prolonged drainage. Furthermore, a new phenomenon of curvature inversion in the raised liquid film center was consistently observed in the late stages of the drainage process, which deviates from existing Reynolds lubrication models and may be attributed to surfactant redistribution and inward circulation resulting from continuous renewal of the liquid film interface. These results demonstrate that the developed apparatus can provide critical parameters involved in the dynamic interaction processes between droplets, laying the foundation for further exploration of the quantitative relationship between inter-droplet interaction forces and film thickness.