Impact of internal flow on particle deposition in a locally heated sessile droplet

A numerical investigation is conducted to study the influence of internal flow on particle deposition patterns in an evaporating sessile droplet. The colloidal suspension droplet evaporates in a pinned mode on an adiabatic solid surface with the wetting zone subjected to localized isothermal heating. The numerical model incorporates the effects of buoyancy, evaporative cooling, and surface tension gradients. The results reveal that internal flow within the droplet significantly influences particle transport and deposition. For a fully adiabatic wetting surface, a clockwise unicellular flow, combined with an outward radial flow at low contact angles, leads to ring-shaped particle deposition. Conversely, for a fully isothermal wetting surface, a counterclockwise unicellular flow during evaporation results in a central bump-shaped particle deposition. When the droplet is locally heated at the periphery of its base, strong flow near the contact line transports some particles to the droplet edge, forming a ring, while the remaining particles circulate within the droplet core, producing a central bump. However, when the droplet is locally heated at the center of its base, the ring at the edge nearly disappears and the central bump becomes more extended and thinner compared to the fully isothermal case.