Viscous to Inertial Crossover in Liquid Drop Coalescence

Abstract

Using an electrical method and high-speed imaging we probe drop coalescence down to 10 ns after the drops touch. By varying the liquid viscosity over two decades, we conclude that at sufficiently low approach velocity where deformation is not present, the drops coalesce with an unexpectedly late crossover time between a regime dominated by viscous and one dominated by inertial effects. We argue that the late crossover, not accounted for in the theory, can be explained by an appropriate choice of length-scales present in the flow geometry.

Short time dynamics of viscous drop spreading

“…However, the values of a necessary to fit our data explore a wider range than predicted for coalescence -in particular in the large viscosity regime where the outer fluid should be negligible. We nevertheless find prefactors that are comparable to other coalescence experiments 17 , so that experimentally the "spreading-coalescence" analogy might still be fully quantitative. Interestingly, the strong influence of the gas was also emphasized for air entrainment by advancing contact lines 29 .…”

“…1(a) is strongly reminiscent to the coalescence of two spherical drops, if we consider the substrate to act as a mirror-plane for the flow. This analogy was first employed by Biance et al 12 , who compared the spreading to the well-studied growth of the neck when two identical drops coalesce [13][14][15][16][17] . This approach has proven very successful in the low-viscosity limit.…”

Comment on “Viscous coalescence of droplets: A lattice Boltzmann study” [Phys. Fluids 25, 052101 (2013)]

“…[2][3][4][5] However, our results obtained for coalescence in a saturated vapor follow r(t)/R 0 ∼ (t/t v ) 1/2 , which is similar to the results observed by GSRV. However, our expansion speed was notably faster than what they observed.…”

“…The authors stated that the reason for these discrepancies is unknown and offered no explanation for the deviation of their results from the linear time evolution of the bridge radius r(t)/R 0 ∼ t/t v reported in experiments. [2][3][4][5] Here we comment on why the results observed by GSRV do not reproduce experimental results.…”

Imprintable membranes from incomplete chiral coalescence

“…In order for the droplets to coalesce, a rupture must occur between two merging interfaces which, coupled with surface tension and hydrodynamic forces, leads to energetic barriers and non-trivial coalescence progression [1][2][3][4][5] . For complex fluids with partial positional and/or orientational order, the complexity of the coalescence reactions increases considerably 6 .…”