This study investigates the motion of a water droplet influenced by a neighboring propylene glycol (PG)-water mixture droplet on a heated substrate, a phenomenon relevant to microfluidics, heat transfer, and surface wetting applications. The experiments were conducted on a glass substrate at temperatures ranging from 24°C to 135°C, with varying PG concentrations (20%−100%) in the actuating droplet. It was found that increased substrate temperatures significantly enhance the droplet velocity, with the highest speeds observed above water's boiling point, where velocities increased by up to 20 times compared to the unheated conditions studied in the literature. However, higher temperatures also limit the droplet's travel distance due to rapid evaporation reducing the surface tension gradient. Also on heated surfaces, droplets can show a chaotic motion and fragmentation, indicating the high surface tension gradient has overcome cohesive forces within the droplet (e.g. at 20% PG). Lower PG concentrations in the actuating droplet also result in higher water droplet velocities due to stronger concentration gradients for room temperature substrates. This work demonstrates the combined effects of substrate temperature and droplet composition on controlling droplet motion, suggesting potential avenues to tune droplet motion behaviors. • Heated substrates can increase droplet velocity up to 20. • Higher temperature reduces travel distance due to evaporation. • Lower PG concentration increases velocity via stronger gradients. • Combined concentration and temperture enable control of droplet motion behavior.
Azzam et al. (2026) studied this question.