pendant droplet, critical volume, simulation, minimum energy, patterned surfaces
In this work, pendant droplets of different liquids (i.e. water, ethylene glycol, methanol, and acetone) on a plain aluminum surface and a three-stripe patterned surface were simulated using the Surface Evolver (SE) software. The critical droplet volume before detachment from a surface and droplet properties such as diameter, height, and surface energy were measured. Surface Evolver is a program that models liquid surfaces shaped by different forces and constraints. The program works by modifying a surface toward minimal thermodynamic energy by a gradient descent method. The initial input to Surface Evolver is a script file containing all the pertinent information about the droplet to be simulated including the gravitational constant, volume, density, and contact angle. For these simulations, the gravitational constant was set to -1 for all liquids (i.e. 9.81 m s-2), and the values of density and contact angle were based on the simulated liquid type. As part of this study, droplet contact angles were measured on an aluminum surface at room temperature (~20°C), and these values were used as inputs in the simulation. The measured static contact angle for water, ethylene glycol, methanol, and acetone were found to be 88.2 ,°61.0 ,°20.9°, and 13.0°, respectively. For each simulation, more than 100 iterations were performed before the droplet geometry converged and was ready for measurement. The surface energy was taken directly after each iteration, while the droplet height and diameter were calculated from simulation pictures using a pixel counting method. The critical pendant droplet volumes for water, ethylene glycol, methanol, and acetone on a plain aluminum surface were found to be 17.9, 21, 19.03, and 14 μL, respectively. For the purposes of this study, the critical volume (i.e. when the droplet starts detaching) was defined when the surface energy was found to be in the range of -0.15 J to 0.15 J. Besides critical volumes, droplet data associated with smaller volumes (i.e. 5, 8, 10, 12, and 15 μL) were also measured and compared among liquids. Patterned surfaces were also studied in this work. These surfaces consisted of a central hydrophilic stripe varying from 1 mm to 5 mm in width sandwiched between two outer hydrophobic regions. Critical droplet volumes on these surfaces are also discussed.