Confocal Raman microscopy inside sessile multicomponent droplets

TL;DR

This paper develops a model for refraction effects in confocal Raman microscopy of evaporating multicomponent droplets, enabling marker-free concentration measurements crucial for understanding droplet dynamics in various applications.

Contribution

It introduces a modified Raman measurement configuration to correct for optical distortions, allowing accurate, marker-free concentration mapping in evaporating droplets.

Findings

Modeling refraction improves measurement accuracy.

Horizontal laser configuration enables analysis near contact lines.

Concentration maps reveal internal droplet gradients.

Abstract

Evaporating multicomponent droplets are ubiquitous and appear in a variety of everyday situations and technological applications, including coating, 3D printing, and energy conversion processes. During evaporation, concentration gradients are typically induced, resulting in flows within the droplets. Many of the mechanisms underlying multicomponent droplet evaporation are not fully understood. However, most methods utilize markers that can be surface active and thus affect droplet dynamics. Thus, high-resolution marker-free measurements of concentration gradients in evaporating multicomponent droplets are needed. Raman microscopy can provide such a method. However, as the Raman laser has to undergo a phase transition, it is refracted, leading to a distorted drop contour. In this study, we model refraction in the droplet to better understand the shift in focus leading to geometric distortion. The horizontal and vertical shifts in focus are analyzed, and a modified configuration that enables Raman measurements with a horizontal laser is introduced. For both configurations, the simulated drop shape in the Raman image fits well with the measured shape. While Raman measurements with a vertically incident laser allow the study of the upper part of the droplet, for droplets with large contact angles, horizontal Raman measurements allow for the analysis of the region around the 3-phase contact line. As an example, a concentration map of an evaporating 4.2 $\mu$L glycerol/water droplet is presented. The results of this study can be used as a guide for Raman microscopy measurements of droplets. These findings contribute to understanding droplet dynamics during evaporation and provide a basis for developing novel printing, cleaning, and energy conversion technology applications.