On the collision of a droplet with a solid surface

Abstract

The collision dynamics of a liquid droplet on a solid metallic surface were studied using a flash photographic method. The intent was to provide clear images of the droplet structure during the deformation process. The ambient pressure (0.101 MPa), surface material (polished stainless steel), initial droplet diameter (about 1.5 mm), liquid (n-heptane) and impact Weber number (43) were fixed. The primary parameter was the surface temperature, which ranged from 24°C to above the Leidenfrost temperature of the liquid. Experiments were also performed on a droplet impacting a surface on which there existed a liquid film created by deposition of a prior droplet. The evolution of wetted area and spreading rate, both of a droplet on a stainless steel surface and of a droplet spreading over a thin liquid film, were found to be independent of surface temperature during the early period of impact. This result was attributed to negligible surface tension and viscous effects, and in consequence the measurements made during the early period of the impact process were in good agreement with previously published analyses which neglected these effects. A single bubble was observed to form within the droplet during impact at low temperatures. As surface temperature was increased the population of bubbles within the droplet also increased because of progressive activation of nucleation sites on the stainless steel surface. At surface temperatures near to the boiling point of heptane, a spoke-like cellular structure in the liquid was created during the spreading process by coalescence of a ring of bubbles that had formed within the droplet. At higher temperatures, but below the Leidenfrost point, numerous bubbles appeared within the droplet, yet the overall droplet shape, particularly in the early stages of impact (< 0.8 ms), was unaffected by the presence of these bubbles. The maximum value of the diameter of liquid which spreads on the surface is shown to agree with predictions from a simplified model.

Splash-cup plants accelerate raindrops to disperse seeds

“…Chandra & Avedisian [59] estimated the magnitude of f to be mðU 0 =dÞ 2 , where d is the boundary layer thickness of the film flow. This thickness, d, was found to be 2ðd= ffiffiffiffiffi ffi Re p Þ by assuming that the fluid flow resembles symmetric stagnation point flow [58].…”

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

“…On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].…”

“…On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U, phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on microstructured silicon [11], FC-72 on carbon nanofiber [12], water on aluminium [13], and ethanol on sapphire [14]. All of these phase diagrams show a weakly increasing behavior of T L with U.…”

“…At low impact velocities, the upper T L boundary is also lower than for ethanol, but it increases with U and reaches almost the same level as that for ethanol at U ≈ 4 m=s. Several models for the spreading diameter D max of an impacting drop have been developed [9,11,29,30]. For the so-called pancake model [11], D max and D 0 =U are considered to be the relevant length and time scales [11].…”

Dynamics of squeezing fluids: Clapping wet hands

“…After impact, a cylindrical sheet forms and moves upwards, eventually creating droplets along the edge of the fluid rim [16]. Liquid crowns of various geometries have been studied extensively; on a bulk of the same fluid [10,17], on a solid wall [18,19], on a thin fluid layer [20,21], on a rod [22,23], and more. The water bell [14,15] is another example sharing some features with our study of clapping with wet hands, i.e., the fluid rim connected to the sheet.…”