Influence of thermalcapillary and buoyant forces on flow characteristics in a droplet on hydrophobic surface

Wetting characteristics of droplets on hydrophobic surfaces are the current interest in many fields because of necessity for self-cleaning, improving lubrication, speedy liquid separation, bacterial activity minimization, reducing fouling, etc. Therefore, we investigate flow and thermal fields in a droplet at a hydrophobic surface due to a localized heating and analyze the effects of droplet contact angle on heat and flow characteristics due to thermocapillary and buoyancy forces developed in the droplet. Trichlorooctadecylsilane coating is introduced on a smooth polycarbonate wafer to generate a hydrophobic surface and a fine sized metallic meshes is laid on the hydrophobic surface where a constant temperature heating is applied at 308 K. Flow filed is simulated numerically incorporating the experimental conditions. A droplet of a nano-fluid, consisting of water and 1% (volume) carbon nanotube mixture, is formed at the hydrophobic surface to monitor the flow velocity in the droplet, which is, then, used for the validation of velocity predictions. It is found that combination of Marangoni and buoyancy forces give rise to formation of circulation cells inside the droplet; in which case, contact angles in the range of 110 degrees <= theta <= 150 degrees two counter rotating circulation cells are formed in the upper part of the droplet. The average Nusselt number increases with increasing droplet contact angle. (C) 2015 Elsevier Masson SAS. All rights reserved.