Contact and interaction between droplet and solid surface is
a fundamental transport phenomena problem, with ubiquitous
presence in various applications. In this paper, we study the
effect of imposing vertical and horizontal ultrasonic vibration on
dynamics and evaporation of sessile droplets of
dimethylformamide (DMF), a pure volatile model solvent.
Droplet contact angle and contact radius are the two main
parameters that may change during evaporation. Hence, droplet
evaporation may be categorized into different modes: constant
angle (CA), constant radius (CR), and a complex combination of
CA and CR modes. Imposing substrate vibration affects the
evaporation rate and mode by changing the thermodynamics and
hydrodynamics of the sessile droplet on the substrate. The former
happens by changing the heat transfer coefficient and the latter
by pinning or unpinning the droplet from the substrate.
Experimental analysis using an optical tensiometer has been
conducted for a small DMF sessile drop on a Teflon substrate.
Among our results, it is observed that the DMF droplet on a
Teflon substrate evaporates in the CR mode until it reaches its
receding contact angle. Then, its contact radius recedes to the
next equilibrium position. Imposing vertical ultrasonic vibration
pins the droplet to the substrate and reduces the receding contact
angle, while horizontal ultrasonic vibration unpins it.
Furthermore, imposing vibration accelerates the evaporation rate
more than 5 times higher than that of the natural convection. The
increase is more significant for the horizontal vibration.
Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .