Contact time (tc) reduction of an impacting droplet on a solid surface is widely needed in many fields. Superhydrophobic (SHB) surfaces are being applied to reduce the tc at room temperature. However, the coating material for inducing superhydrophobicity on an SHB surface may be damaged at high temperatures, depriving the tc reduction ability of an SHB surface at high temperatures. Furthermore, when the temperature is higher than the Leidenfrost point (LFP), a stable vapor cushion forms on an SHB surface, insulating the solid surface and the liquid droplet. Consequently, the heat and mass transfers between the solid surface and the droplet are inhibited due to the LF effect. The LFP on a solid surface decreases with increasing hydrophobicity of a surface and an SHB surface usually exhibits a low LFP. Therefore, a surface that is able to suppress the LF effect and reduce contact time at high temperatures is required.In this work, a double reentrant groove (DRG) array surface (named as DRG surface hereafter) was proposed to tackle this challenge. This DRG surface possesses an overhanging structure composed of the nanoscale horizontal and vertical overhangs on top of the microgrooves. Thus, the DRG surface features an upward surface tension force to resist the impacting droplet from penetrating the microgrooves. The contact angle on the DRG surface is ~141°. The droplet bouncing dynamics on the DRG surface was studied and compared to that on the single reentrant groove (SRG) array and microgroove (MG) array surfaces (named as SRG and MG surfaces, hereafter). The SRG surface consists of only the horizontal overhanging structure on top of the microgrooves and the MG surface has no overhanging structure on top of the microgrooves. The measured LFP on the DRG, SRG, and MG surfaces were in-between 500~530 °C, 500~530 °C, and 450~475 °C, respectively. The contact angles on the SRG and MG surfaces were ~87° and ~63°, respectively. The LF effect was suppressed on the DRG surface despite that the DRG surface is hydrophobic. The LF suppression of the DRG surface resulted from the blockage of the upward vapor flow by the overhanging structure. The upward vapor flow was hindered by the overhanging structure and the vapor flow escaped more effortlessly along the microgroove direction. The LFP on the DRG was elevated due to the pressure loss resulting from the vapor escape.&nbsp;Between 350 to 530 °C, the DRG surface gave the largest dimensionless maximum droplet diameter (Dmax/Do) and smallest tc) among the three surfaces. This is presumably because that the DRG surface had the largest boiling area and created the maximum asymmetric momentum force. The&nbsp;Dmax/Do&nbsp;increased with temperature and reached a maximum at 477 °C on the DRG surface. Compared to&nbsp;Dmax/Do&nbsp;­, the&nbsp;tc&nbsp;exhibited an opposite trend against the temperature. The increase of&nbsp;&nbsp;Dmax/Do&nbsp;­and the reduction of&nbsp;tc&nbsp;with temperature between 350 to 477 °C were due to the increased boiling-induced asymmetric momentum force with temperature. Further increasing the temperature from 477 °C resulted in the increase of vapor coverage ratio on the surface and the boiling-induced asymmetric bubble momentum force reduced. Thus,&nbsp;Dmax/Do reduced and&nbsp;tc&nbsp;increased, respectively, with temperature afterward. The DRG surface is the only hydrophobic surface that can be applied to reduce the droplet contact time at high temperatures found in the literature. Compared to the results on the SHB surface at room temperature in the literature [1], a 400% increment of Dmax/Do ­and a 26% reduction of&nbsp;tc&nbsp;were obtained on the DRG surface at 477 °C.&nbsp;Reference:1.&nbsp;&nbsp;&nbsp;&nbsp; Clanet, C.;&nbsp; Béguin, C.;&nbsp; Richard, D.; Quéré, D., Maximal deformation of an impacting drop. J. Fluid Mech. 2004, 517, 199.

Leidenfrost Suppression and Contact Time Reduction on the Double Reentrant Groove Array Surface

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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