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| Dynamics of impacting droplets |
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 Heat transfer and droplet dynamics on surface enhanced surfaces
Interfacial phenomena are strongly affected by surface topography. Great interest has been shown by the researchers in the use of micro-textured surfaces with potential application in a wide range of situations, as for instance in the development of integrated cooling systems based on spay/droplet impact, for thermal management of specific areas or small electronic parts. In the present work, a systematic approach is followed where various design parameters (e.g. liquid, properties, droplet diameter and impact velocity, impaction angle) and boundary conditions related to the nature of the surface are varied to infer on their effect on the thermo-fluid-dynamic behaviour of the droplets. Droplet behaviour is characterized by combining high-speed imaging with phase Doppler measurements, while the thermal behaviour is studied based on the temperature and flux evaluation, based on the temperatures measured at the impact region, making use of fast response thermocouples. The micro-textured surfaces are developed to have well defined topographical properties in terms of roughness amplitude (quantified by the mean roughness Ra and average peak-to-valley roughness, Rz) and fundamental wavelength, lR (distance between rough peaks), as illustrated in Figure 1.
Figure 1. Topographical parameters used to characterize the micro-textured targets
Preliminary results evidence that micro-textured surfaces can be used to enhance the cooling performance of the impacting droplets, but an optimization of the topographical parameters has to be performed, based on the relations between Ra/lR in a compromising solution of endorsing liquid-solid contact without promoting an ex excessively intense thermal induced atomization. For instance Figure 2 depicts the surface temperature variation, at the impact region, for water droplets impinging onto micro-textured stainless steel surfaces, within the bubble boiling regime. Both thermal and fluid dynamic behaviours depend on the relation Ra/lR, but not in a monotonic way. Hence, secondary atomization is triggered earlier for the textured surface with Ra/lR =0.04. Moreover, the temperature decay is also larger for this target. This suggests that larger relations obtained with the other targets are endorsing a heterogeneous wetting phenomenon onto the heated targets.
Figure 2. Effect of using micro-structured surfaces on the surface temperature variation, measured at the point of impact, for a water droplet (D0=2.8mm, U0=1.3ms-1) onto stainless steel surfaces, within the bubble boiling regime (DTsuperheat=40ºC).
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