On the role of liquid viscosity in affecting droplet spreading on a smooth solid surface

The role of liquid viscosity on droplet spreading behavior upon impacting on a smooth stainless steel surface has been experimentally investigated. Results show the droplet spreading dynamics with increasing viscosity (characterized by the Ohnesorge number, Oh)exhibits complex dependence on the impact inertia (characterized by Weber number, We). Specifically, for a small impact inertia (We<30), the droplet oscillates in the vertical direction around the maximum height H_a. The non-dimensional maximum diameter β_max first increases and then decreases with increasing Oh, and this non-monotonic phenomenon has not been reported previously. For an intermediate impact inertia (60<We<240), the droplet has no oscillation after it spreads to β_max, and it has the form of a rim-bounded lamella. Although β_max shows a monotonic decrease with increasing Oh, some unsmooth disturbance around the rim occurs only at intermediate Oh. For a higher impact inertia (We>240), droplet splashing emerges and then vanishes with increasing Oh, although β_max still decreases monotonically. All the observed phenomena imply that liquid viscosity may have a dual role in affecting the droplet spreading, which previous models of β_max do not take into account.