Effect of temperature gradient on the substrate on the coffee-ring dimensions

We experimentally investigate the coffee-ring deposits obtained after evaporation of a 1.1 µl water droplet containing polystyrene colloidal particles (0.3 µm diameter) with an imposed temperature gradient on the surface of the glass substrate. The temperature gradient was achieved by using two thermoelectric coolers underneath the substrate and was confirmed by the infrared thermal camera measurements. High-speed visualization was employed to record the time-varying droplet evaporation. The three substrate temperature gradients obtained were 3.5 °C/mm, 2.4 °C/mm, and 1.3 °C/mm. The deposits are qualitatively visualized under an optical microscope, and scanning electron microscope and are compared with the deposits obtained on constant temperature substrate. The measurements showed that there is a substantial difference in the ring width on the hot side ring perimeter than the cold side ring perimeter. Due to the imposed temperature gradient, the hot side droplet is at lower surface tension than the cold side and the Marangoni convection loop changes inside the droplet into a single loop unlike the two loops on a constant temperature substrate. The fluid and the particles flow from the hot side to the cold side along the droplet surface.