Previous studies have pointed out that adding a single protrusion on the superhydrophobic surface can effectively reduce the contact time of droplets, which is of practical importance in applications like anti-icing, self-cleaning, and anti-frost. However, the droplet impact dynamics and the mechanism for contact time reduction are still far from completely understood. Therefore, in this paper, via a three-dimensional pseudopotential lattice Boltzmann model coupled with a modified curved boundary scheme (which satisfies mass conservation), the droplet impact dynamics is simulated in a wide range of Weber numbers (0 < We≤ 67.7), protrusion size (10 ≤ W ≤ 30), and protrusion shape (triangle, square, and circle), with particular interest in understanding their effects on the contact time. We demonstrate that the variation of contact time depends on the bouncing type as well as the retraction way. Among three possible bouncing types observed in the simulation, including non-break bouncing, two droplets bouncing, and three droplets bouncing, two droplets bouncing has less contact time and could be facilitated by increasing We and using triangle protrusion. However, with increasing of W or using square protrusion, the bouncing type may change from two droplets bouncing to three droplets bouncing, and the deformation of fragments may become definitely different, leading to a long contact time. In addition, a transition from twice-retraction to once-retraction can be obtained by increasing We or making the protrusion shape sharp. When the fragments resulting from droplet splitting only retract once on the bottom wall, the contact time can be effectively reduced.

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This work proposes an improved three-dimensional double multiple relaxation time lattice Boltzmann model that can correctly recover the macroscopic temperature equation. Using this model, the impact of a droplet on a heated surface with pillars is simulated, and the effects of pillar width (w) and distance between pillars (b) are analyzed in detail. Four boiling droplet types are numerically reproduced, namely, deposition boiling, contact boiling, transition boiling, and film boiling. In the deposition boiling regime, no vapor bubble generation is observed within the droplet, but secondary droplets are generated at the periphery of the droplet due to the sharp geometric profile of the pillars and the differences in the rate of evaporation. The difference between contact boiling and transition boiling lies in whether the droplet is always in contact with the surface. For conditions with large w values, contact boiling is not observed, due to the high-pressure peaks from below, while for smaller w, a transition from deposition boiling to transition boiling, then to contact boiling, and subsequently to transition boiling can be observed. Two obvious vortices are found in the center of the liquid film with decreased b, which results in bubble nucleation even at low Ja values. In addition, boiling may take place, in which a stable vapor layer is formed between the droplet and the surface by increasing Ja, which leads to a significant reduction in the heat transfer efficiency.

HighlightsThe performance of biomimetic rollers was better than that of a conventional roller.The biomimetically ridged roller had better soil compaction performance.The resistance order of rollers with different materials was: Fe360A steel > UHMWPE > enamel coating.Packing force was the most significant factor.Abstract. To reduce soil adhesion and rolling resistance during seeding and improve the uniformity of plant spacing, a biomimetically ridged press roller (BRPR) and a biomimetically polyhedral press roller (BPPR) were designed. Moreover, hydrophobic materials, i.e., ultra-high molecular weight polyethylene (UHMWPE) and enamel coating, were applied and compared with uncoated Fe360A steel. Field experiments were conducted to test the effects of the biomimetic press roller type, material, velocity, and packing force on soil compaction characteristics. A conventional press roller (CPR) was used as a reference. Traction resistance (TR), soil bulk density (SBD), soil moisture content (SMC), emergence rate (ER), and percent change of plant spacing (PCPS) were measured. The packing force was the most significant factor affecting each index. For all rollers, the TR, SBD, ER, and PCPS increased with increasing packing force, while the loss of SMC decreased. The TR and SBD decreased with increasing velocity. All rollers achieved proper SBD for corn seeds. The biomimetic press rollers had a significant effect on TR, PCPS, and SMC on the third day after sowing. The BRPR had lower PCPS and TR than the BPPR and higher SMC than the BPPR on the third day after sowing. Compared with the CPR, the PCPS for all biomimetic rollers decreased, and the ER and SMC increased. The BPPR with Fe360A steel had a slightly higher TR than the CPR, but the other biomimetic rollers had a lower TR than the CPR. The BRPR decreased the TR by 2.13% to 22.30%, while the BPPR decreased the TR by 0.35% to 18.59%. Moreover, both types of biomimetic press rollers had the highest TR values when using Fe360A steel, followed by UHMWPE, and the enamel coating had the lowest TR values. The BRPR decreased the PCPS by 19.46% to 48.34%, while the BPPR decreased the PCPS by 9.69% to 36.99%. The biomimetic rollers increased the ER by 2.17% to 11.36%. On the 18th day after sowing, the SMC was 4.21% to 7.75% higher for the biomimetic press rollers compared to the CPR when the packing force was 700 N. This study can provide a reference for the biomimetic design, material, and force selection of press rollers. Keywords: Biomimetics, Compaction characteristics, Enamel coating, Press roller, UHMWPE.