Numerical Investigation Of Coalescence-Induced Droplet Jumping From A Hydrophobic Fiber

Under certain conditions,droplets can merge on superhydrophobic surfaces and the merged droplet can jump off the surface due to the transformation from surface energy to kinetic energy.The study of this phenomenon will have a positive impact on improving condensation heat transfer efficiency,self-cleaning surfaces,anti-icing surfaces and fog collection.At present,a lot of research results have been obtained on the phenomenon of spontaneous jumping of droplets.These results include not only experimental results but also theoretical analysis,as well as numerical simulations.Most existing studies focused on flat superhydrophobic surfaces and the studies on droplet coalescence on curved surfaces are relatively rare.Here we carried out numerical study of coalescence-induced droplet jumping on a commonly seen curved surface—fiber surface,under different conditions.In this thesis,we employed a phase-field-based hybrid lattice-Boltzmann finite-difference method in which the interface dynamics is handled by the finite-difference solution of the Cahn-Hilliard equation and the hydrodynamics is handled by the lattice-Boltzmann method.The two groups of equations were coupled through the convection term in the interface evolution equation and the surface tension term in the fluid momentum equation.For wetting boundary conditions of a curved surface,we used a recently proposed new characteristic interpolation method.We first fixed the Ohnesorge number at Oh=0.0126,and explored the influence of the radius ratio of droplets to fibers and the wettability conditions(i.e.contact angle)on the surface of fibers.Then,the influence of Ohnesorge number on droplet jumping was explored.Besides,we also investigated the effect of Cahn number in phase-field simulations on the results.It was found that at a small Ohnesorge number,several different outcomes,including normal jumping at a positive velocity,jumping with a negative velocity,jumping after wrapping the fiber,and oscillating on the fiber,may occur after droplet coalescence,depending on the wettability of the fiber and the droplet-to-fiber radius ratio.It was found that only when the contact angle and radius ratio between droplets and fibers exceed a certain critical value can the droplet jump off the fiber.Moreover,the critical radius ratio decreases as the fiber becomes more hydrophobic.For a given contact angle and radius ratio,it was observed that there exists an Ohnesorge number at which the jumping speed reaches its maximum.Some of these findings confirm(numerically reproduce)previous reports,and some are reported for the first time.We also analyzed the process of droplet coalescence and jumping on fibers under different conditions from the view of the evolutions of various energies.It is expected that this study could extend our knowledge on the coalescence-induced droplet jumping from a hydrophobic fiber and provided meaningful reference for related applications.