The study performed under this task was aimed at accounting for the complexity introduced by the influence of non-scaled parameters on the description of droplet impact. An experimental installation was built where droplets are generated at the tip of a hypodermic needle and fall by gravity onto a cold and flat surface. The time history of droplet behaviour is recorded with a high-speed camera triggered by the passage of the droplet through a laser beam hitting onto a photodiode. The experiments included the use of several targets made of dissimilar materials, from polished surfaces to strong rough surfaces with different roughness profiles, namely roughness wavelength and shape of the asperities. Surface roughness was characterized with an optical profile meter for opaque surfaces and with a mechanical profile meter for transparent surfaces. Numerous measurements were performed along orthogonal directions in order to form a grid of small target areas with well-defined roughness characteristics. Inhomogenities of the targets, characterised by variations of the mean roughness, were found to be smaller than 10%.
The thermodynamic system liquid-vapour-surface was characterized by the equilibrium contact angle, as provided by Youngs law, as a measure of the wetness. Contact angles at equilibrium were measured for several rough surfaces and liquids. The measurements were made with sessile drops inside a thermostat ambient chamber (Ramé-Hart Inc., USA, model 100-07-00), with quartz windows, to avoid optical distortion, previously saturated with the liquid to be studied (water and diesel oil) at a temperature of 20 ±1ºC. After the drop was deposited at the surface, its image is recorded using a colour video camera (JVC Colour TK-1070) mounted on a Wild M3Z microscope, which allows a magnification of 40 times. The video signal was transmitted to an image processor - Video Pix Framegrabber (Sun Microsystems) - and digitized in 640 x 480 pixels images, in a 256 grey level scale. The image acquisition and analysis was performed by a Sun Sparc station IPC, using the Axisymmetric Drop Shape Analysis software (ADSA for SunOS 1.0 Applied Surface Thermodynamics Research Associates, Toronto, Canada). The variation of the contact angle with time was recorded for time intervals of six hundred seconds. At least 8 measurements were taken for each pair liquid-impact surface in order to obtain average values. Furthermore the evolution of the average contact angles with time was obtained for each pair liquid-surface, by curve fitting and the final values were determined by extrapolation. Since the outcomes from droplets impact occur in a much smaller time scale (ms) it was considered that the final value for the contact angle was obtained by fitting the values obtained in the latest period of the measurement and extrapolating them for t = 0s.
Experiments included the identification of mechanisms and an attempt to describe the effects of the target surface on them. The results showed that the wetness strongly depends on the mean surface roughness, and may be used as a characteristic parameter of the system, providing that a precursor film is not formed at the target surface during spread. If splash does not occur, the equilibrium contact angle describes the effects of surface roughness on the wetting dynamics. Those effects are expected to alter friction forces at the wall and, therefore, the energy dissipated during spread. It is found that, provided the Reynolds number of the droplet at the impact is large (Re>2000), the energy dissipated at the wall is not greatly affected by the nature of the surface, which appears to be important for low Reynolds numbers, typically Re<1000.
However, the nature of the surface significantly alters the onset of splashing. The correlation of Wu (1992) was observed to correlate the dimensionless roughness Ra/Ro with the critical Weber number at the onset of splash for each pair surface material - liquid. The accounted properties of the surface do not allow collapsing all the curves and other characteristics should be taken into account, such as the wavelength of the irregularities.