Drop spreading and penetrating on micro/nano particle sintering porous with multiscale structure

Drop spreading on micro/nano particle sintering porous is a challenge issue due to multiscale behavior. Here, the 556 nm or 16 mu m copper powders are sintered with or without pore former, under vacuum or oxidation environment to generate twelve porous substrates, having two to three levels of length scales from nano to micron (10-100 mu m). Drop spreading experiment was performed with careful data processing using time sequence images. It is found that early inertia drop spreading on porous surface follows the well-known power law. Nano-roughness increases wettability to increase the power-law coefficient C to accelerate spreading for both nano and micro-particle sintering surfaces. The porosity does not change the power law exponent a for nano-particle sintering surface, but the increased porosity decreases both C and a for micro-particle sintering surfaces. Nano-particle sintering porous lasts longer duration time of inertia dominant spreading, during which the first time daughter drop emission is success. Micro-particle sintering porous shortens the duration time. The pinching neck is still thick at the end of the duration time. Thus, the first time daughter drop emission is not success. This work identified the necessary condition for the first time daughter drop generation is that the pinching phenomenon should occur during the inertia dominant spreading stage. Spreading, capillary wave and penetration are competition mechanisms to form the S-shape pinching neck heights. The re-increase of the neck heights evidences the secondary capillary wave propagation, corresponding to the second time daughter drop emission. The secondary daughter drop emission time (t(sp)) is either shorter or longer than the maximum spreading diameter time t(M), inferring that the secondary daughter drop generation is not related to the porous structure. (C) 2016 Elsevier B.V. All rights reserved.