CO2-driven diffusiophoresis in an evaporating sessile droplet

The ability to control particle transport within evaporating droplets is important for a broad range of printing applications. However, it remains challenging to modulate the complex multiphase phenomenon to create high- quality thin films. For example, evaporation-induced capillary flows in a pinned droplet can propel particles toward the contact line, forming a characteristic ring-like pattern (also known as the "coffee-ring effect"). Previous work has shown that introducing temperature or surface tension gradients can generate Marangoni flow, which at sufficiently high magnitude can redirect the particle assembly. Here, we present an alternative approach to manipulate particle transport during evaporation via CO2-driven diffusiophoresis. Specifically, we compare the internal flows and particle transport within evaporating droplets with capillary- and Marangoni-dominant flows in the distinct environments of air or CO2 through simulations and validate the diffusiophoretic effect on the droplet deposition pattern via experiments. We found that the diffusiophoretic particle motion can dominate capillary flow, leading to particles' migration towards or away from the droplet surface as determined by their surface charge. Further, we learned that in the presence of temperature gradients, Marangoni flows can overwhelm diffusiophoresis by saturating ions in a short time. The CO2-driven diffusiophoresis can modulate final deposition patterns by influencing particle motion during the evaporative-driven assembly process. This study provides a more comprehensive and clearer understanding of the fundamental physics on how diffusiophoresis interacts with internal flows in evaporating droplets. We highlight its capability to control deposition patterns with minimal solution contamination and a simpler setup compared to previous approaches.