Suppression of Coffee-Ring Effect in Droplets with Varying Particle Concentrations Induced by Laser.

The coffee-ring effect commonly occurs during the evaporation and deposition of particle-laden droplets on the substrates. The resulting deposition pattern is influenced by internal flows, which depend on substrate properties, droplet chemistry, and external conditions. However, in applications such as inkjet printing, drug formulation, and self-assembly, avoiding coffee ring formation is crucial. This study presents a method to suppress the coffee-ring effect using focused laser irradiation on droplet surfaces. By irradiating droplets containing silica microparticles with a green laser and adjusting the particle concentration, we controlled the deposition patterns. Uniform patterns formed at particle concentrations below 0.02%, while gradient patterns appeared above 0.08%. A dimensionless parameter, , was introduced to quantify particle deposition patterns. Uniform deposition structures are established when > 10, whereas gradient structures are formed when 2. This approach offers new insights into the laser-induced control of microparticle deposition patterns.