3D holographic imaging of the evolution of internal flow and particle transport in an evaporating droplet

The internal flow motion in an evaporating droplet serves as one of the key driving mechanisms that lead to the variety of particle deposition patterns in applications such as inkjet printing, forensic analysis of blood stains, and surface patterning. Using digital inline holography (DIH), we examine the 3D internal flow for different types of droplets and the evolution of such flow fields, and corresponding particle transport throughout the entire evaporation process. Our results reveal the presence of different regimes during internal flow evolution associated with variation in the relative significance of evaporation-driven flow effect, Marangoni effect, and boundary movement and some unique 3D flow features such as the change of the extent of Marangoni flow in the 3D space and vertical fluid motions. By tracking the movement of numerous particles from beginning to deposition, our study suggests the shift of dominance of the three effects has direct impact on final particle deposition pattern. Based on our experiment results, an analytical model incorporating the effects of evaporation-driven flow, Marangoni stresses, and boundary movement is developed and can provide a reasonable prediction of internal flow and particle deposition patterns for a variety of Newtonian droplets.