Observation of colloidal particle deposition during the confined droplet evaporation process

A circular silicone sheet as a masker was used to cover a glass slide, and then the super-hydrophobic coating was sprayed on the glass slide free of silicone sheet masker, thus a round hydrophilic area surrounded by a super-hydrophobic coating is obtained. The PS colloidal droplets are confined in the hydrophilic area, and the droplet volume can be changed within a large range. Variation of the droplet volume influences the initial apparent contact angle. We investigate the particle deposition behavior of the confined colloidal droplet for a hydrophobic apparent contact angle in evaporation process by using an in situ optical observation system. In the whole evaporation process the contact-line of the confined droplet is pinned at the junction between the hydrophilic area and hydrophobic area. In the particle deposition process the main driving flow is different, and the final deposition pattern is controlled by three flow behaviors. In the early stage, the main flow is the Marangoni flow, which drives the particle clusters float on the droplet surfaces, part of them accumulated at the boundaries. As the evaporation proceeds, when the apparent contact angle decreases (< 60 degrees), the evaporation flux becomes singular near the contact line, Capillary flow towards the contact inside the drop as a compensation to the solvent loss at the drop boundary, which drives the particles in the droplet to rapidly accumulate at the contact-line. In the last evaporation stage, the thickness of the film in the hydrophilic area becomes very thin, and there is only one layer of particles in this thin film, the thin liquid film instability triggers the particles in the middle area to rapidly aggregate and then form a kind of network pattern, due to the decrease of distances between the particles. Capillary force between particles also takes part in this aggregate process.