Experiments in Fluids

, Volume 22, Issue 6, pp 463–472 | Cite as

The impact of a single drop on a wetted solid surface

  • G. E. Cossali
  • A. Coghe
  • M. Marengo
Article

Abstract

The impact of single drops on a thin liquid film was studied to understand the mechanism of secondary atomisation of sprays colliding on a wetted, cold, solid surface. To span a wide range of conditions various mixtures of water and glycerol were used. The use of Weber number, Ohnesorge number and non-dimensional film thickness to describe the peculiarities of the phenomenon allowed to carry out the experiments under appropriate similarity conditions. The impact of millimetric drops was analysed in detail by photographic means, using both still photography to study impact morphology, and laser sheet visualisation to investigate secondary droplet formation. Two mechanisms of splash were identified, depending essentially on the liquid viscosity (Ohnesorge number), a parameter which appears to play an important role also in defining the splash morphology. A photographic documentation is annexed. The characteristic times of the crown formation, the non-linear evolution of cusps (jet formation) and the surface roughness influence are further discussed. The experimental results allow to propose an empirical correlation for the splashing/deposition limit, for a wide range of conditions, and a comparison to available previous works is presented. The influence of the film thickness and liquid viscosity on the splash is confirmed and quantified.

List of Symbols

Bo

Bond number (= ρgh2/σ)

Ca

capillary number (= µV/σ)

D

nozzle diameter

f

impact frequency

fnd

non-dimensional impact frequency (=f φ/V)

Fr

Froude number (= V2φ/g)

h

film thickness

H

crown height

K

We Oh- 0.4

KL

splashing/deposition limit

N

number of secondary droplets

Njet

number of jets detaching from the crown

Oh

Ohnesorge number (= µ/ φσρ 1/2)

Ra

roughness

Rc, nd

nondimensional crown radius (= Rc/φ)

Rc

crown radius

Re

Reynolds number (= σVφ/µ)

Rnd

non-dimensional roughness (= Ra/φ )

t

time

ts

splash beginning time

Ug

gravitation potential energy

Us

surface potential energy

V

terminal drop velocity

We

Weber number (= ρV2φ/σ)

Wecr

critical Weber number

Wed

deposition Weber number

Wes

splash Weber number

Y

splashing/deposition parameter

Greek symbols

α

drop impact angle

δ

non-dimensional film thickness (= h/φ)

φ

drop diameter

Φ

coalescence parameter

φjet

jet diameter

λ

viscosity length ( = (µ/αf)1/2σρ/µ2)

µ

liquid viscosity

ρ

liquid density

σ

surface tension

τ

non-dimensional time (= t φ/V)

τ0

splash time scale ( = φ/V)

τs

nondimensional splash beginning time (= ts0)

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Copyright information

© Springer-Verlag 1997

Authors and Affiliations

  • G. E. Cossali
    • 1
  • A. Coghe
    • 2
  • M. Marengo
    • 2
  1. 1.Universita di BergamoDalmine, BergamoItaly
  2. 2.Politecnico di MilanoMilanoItaly

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