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The impact of a single drop on a wetted solid surface

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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.

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Abbreviations

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

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 (= ts/τ0)

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Authors and Affiliations

  1. Universita di Bergamo, Via Marconi, 5, 1-24044, Dalmine, Bergamo, Italy

    G. E. Cossali

  2. Politecnico di Milano, P. za Leonardo da Vinci 32, 1-20133, Milano, Italy

    A. Coghe & M. Marengo

Authors
  1. G. E. Cossali
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  2. A. Coghe
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  3. M. Marengo
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Additional information

Milan. The authors would like to thank Mr. Valentino Michelotti and Mr. Stefano Milza for their work during the experiments. Mr Gianni Brunello for his assistance, Ing. Bollina for the surface treatment and roughness measurement of the aluminium disk. We are also indebted to Prof. Cam Tropea and Prof. A.L. Yarin for many helpful and stimulating discussions.

The experiments were performed at CNPM-CNR laboratories in Milan. The authors would like to thank Mr. Valentino Michelotti and Mr. Stefano Milza for their work during the experiments. Mr Gianni Brunello for his assistance, Ing. Bollina for the surface treatment and roughness measurement of the aluminium disk. We are also indebted to Prof. Cam Tropea and Prof. A.L. Yarin for many helpful and stimulating discussions.

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Cossali, G.E., Coghe, A. & Marengo, M. The impact of a single drop on a wetted solid surface. Experiments in Fluids 22, 463–472 (1997). https://doi.org/10.1007/s003480050073

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  • DOI: https://doi.org/10.1007/s003480050073

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