A Mechanistic Model for Bacterial Retention and Infiltration on a Leaf Surface during a Sessile Droplet Evaporation

Evaporation of sessile droplets on the surface of plant leaves is a process that frequently occurs during plant growth as well as postharvest processes. Evaporation-driven internal flows within sessile droplets can transport microorganisms near the leaf surface, facilitating their adhesion to surface microstructures such as trichomes, and infiltration into available openings such as stomata and grooves. A mechanistic model for this retention and infiltration pathway was developed. Solution domain is a sessile droplet located on a leaf surface, as well as its surrounding gas. The model includes fluid flow within the droplet and gas phases, gas-water interface tracking, heat transfer, transport of vapor in gas, and transport of sugar and bacteria within water. The model results are validated based on available literature data and experimental images. The results showed that a hydrophilic surface would promote bacterial retention and infiltration. Evaporation-driven flows increase concentration of bacteria around or inside microstructures at the leaf surface, facilitating their adhesion and infiltration. Larger microstructures having wider spacing between them increased the retention. A higher evaporation rate led to higher infiltration. Chemotaxis toward nutrients at the leaf surface and random motility were shown to decrease the retention and infiltration during evaporation.