An experimental investigation of fluid flow resulting from the impact of a water drop with an unyielding dry surface

Abstract

The flow of fluid associated with the impact of water drops of radius R at a speed V onto unyielding dry metal surfaces of known roughness R a is described. Spatial dimensions of the deforming drop are normalized by transformations of the kind x ' — x/R , and time scales are normalized according to t ' = tV/R , to permit comparison of events where or differ. It is shown that the primary influence of the surface roughness parameter R a is the determination of the condition for the ejection of secondary droplets by the excitation of an instability in the developing watersheet; provided R a ≪ R , it is possible to evaluate the condition to a high degree of accuracy, and for R a = 0.84 μm it is found to be α4/3 RV 1.69 > 7.4, where α is the eccentricity of the drop at the moment of impact. Deceleration of the drop apex does not commence until > 0.6, contrary to the prediction of Engel (1955) but in good agreement with that of Savic & Boult (1957). Close examination of the very early stages of impact suggests strongly that the so-called watersheet originates at a moment t ' — 0.01 after first contact, regardless of the absolute values of R, V or R a ; the initial normalized watersheet velocity is of order 5. Where there is ejected material, its normalized velocity at the moment of ejection is of the order of 20 % greater than that of the watersheet substrate. Simple calculations also suggest that initial fluid velocities greater than 10 are required immediately before the initiation of the watersheet ( t '< 0.01). Impacts at speeds considerably greater than the appropriate terminal fall speed in air show no deviations in character from those investigated at much lower speeds. A simple subsidiary experiment also suggests that greater impact velocities are required to produce splashing on inclined targets.

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

“…A rich body of literature already exists considering perpendicular drop impacts, with a droplet falling from above onto a flat surface [1][2][3][4][5][6][7][8]. These studies have revealed the underlying mechanics and possible outcomes of droplet splashing and are industrially relevant to processes such as spray coating, spray cooling and inkjet printing.…”

“…For droplet impacts at a normal angle of incidence, the threshold of the onset of splashing (K) has traditionally been defined using the Weber number, = 2 / , and the Reynolds number, = / , with μ the dynamic viscosity of the liquid. The splashing threshold (K) has been shown to follow the correlation [5,16]:…”

Fluid dynamics topics in bloodstain pattern analysis: Comparative review and research opportunities

“…At even faster flow rates dripping transitions towards jetting, when the Weber number reaches a critical value [67]. Also, dripping and jet breakup induce longitudinal oscillations in the generated drops [68,69]. The frequency of these oscillations is controlled by surface tension and the mass of the drop [70], and viscous forces tend to dissipate these oscillations.…”

“…Area of Convergence, Directionality [13] [ 5,97] Area of Origin [13,19,68 Backspatter Pattern; Forward Spatter Pattern area of origin, weapon [48] [5, 47,110,191] Blood Clot [6,162] [ 160 , 162] Bloodstain pattern area of origin, weapon [13,35,76 [ 166,170,196] Perimeter Stain time between drip and wipe-off, [155,197,198] [ 155,188] [188] [155,163,197,198] Flow Pattern [12,162,199] [12, 162,185,199] [185] [12] Impact Pattern weapon, motion; directionality [78][79][80]128] [5] [72,128,136] Insect Stain [157,200] Mist Pattern weapon, directionality [81, 154 , 186 Spatter Stain [201] [126, [15...…”

Adhesion of tin droplets impinging on a stainless steel plate: effect of substrate temperature and roughness

“…Stow and Hadfield [6] studied the effects of surface roughness on spreading and splashing of water droplets and established that splashing was promoted by increasing drop diameter (D), impact velocity (V), and surface roughness (R a ). These droplet impact parameters were combined with the density (r), viscosity (m), and surface tension (s) of the liquid to give two non-dimensional groupings: the Reynolds number (Re ¼ rVD=m) and Weber number (We ¼ rV 2 D=s).…”