Probing the Colloidal Particle Dynamics in Drying Sessile Droplets.

Particle deposition and assembly in the vicinity of contact lines of evaporative sessile droplets have been intensively investigated during the past decade. Yet little is known about particle arrangement in the contact-line region initiated by the self-assembled particles at the air-liquid interface and how the particle pinning behaves differently compared with that when particles are transported from the bulk of the sessile droplet to the three-phase contact line. We utilized the dual-droplet inkjet printing process to elucidate the versatility in particle deposition and assembly generated near the contact-line region and demonstrated the influence of such printing technique on particle pinning at the contact line after solvent evaporation. Wetting droplets containing sulfate-functionalized polystyrene (sulfate-PS) nanoparticles were jetted over the supporting droplets with carboxyl-PS nanoparticles, where the interplay between the solvent evaporation and particle transport dictates the final morphology of particle deposition. Depending on the particle size and concentration used in the supporting droplet, different morphologies of particle depositions near the periphery of the supporting droplet have been obtained such as stratified rings, blended rings, and rings of particles mainly from the air-liquid interface. Three characteristic times are considered in this study, namely, total time for solvent evaporation ( t), time required for the colloidal particles in the supporting droplet to reach the contact line and form the first layers of deposition ( t), and time needed for the particles at the interface to reach the contact line ( t). The ratios of characteristic times ( t/ t) and ( t/ t) determine the final particle assembly near the contact-line region. The ability to control such particle deposition and assembly could have a direct implication on developing facile, cost-effective technologies essential for patterning heterogeneous structured coatings and devices.