Spread and recoil of liquid droplets impacting on solid surfaces with various wetting properties

Thermally sprayed coatings are manufactured by molten or semi-molten droplets impinging onto the target substrate. The properties of coatings created (porosity, conductivity, cracks, ...) depend largely on the dynamics of the droplets which is an outcome of joint aspects, such as properties of liquid droplet and surface wettability. The present work investigates the normal impact of liquid droplets (molten ceramic or metallic droplets) onto dry solid surfaces of various wetting properties. Use is made of the Volume-Of-Fluid (VOF) interface tracking method with a single set of mass and momentum conservation equations. The wettability of a solid surface, characterised by a contact angle, is taken as part of the boundary conditions in the Smooth VOF algorithm. Introducing the dimensionless time t* = tV 0/D0 for a droplet with the initial speed V 0 and initial diameter D0, its spread factor ξ (= D/D0) is found to be proportional to the square root of t* when 0.2 < (t*)1/2 < 0.35. There exists a transitional zone where the changing rate of the spread factor decreases with the ascending contact angle. In addition, the wetting effects on the spreading can be ignored when 0.2 < (t*)1/2 < 0.35 but have to be accounted for once (t*)1/2 < 0.2. Further, the droplet jetting time is found to be inversely proportional to the impact velocity but not affected by the solid surface wettability. Finally, in the recoil stage, the influences of Weber number, Ohnesorge number and contact angle are studied; in particular, the spread factor ξ turns out to be well described by a second-order polynomial equation for t*≥0.5We.