Mechanical Study On Wetting Behaviors Of Water Droplet On Nano/Micro Structured Smooth Surface

Wetting behaviors study of water droplet on nano/micro-structured and atomicsmooth surfaces is a very important topic in the field of fluid interface, and recently ithas been attracting widely attention and has wide range of applications. However,effect of size, topology of the structured substrate and curvature were usually ignoredin the traditional view, so people could not get clear understanding about the wettingbehaviors of water droplet. This work studied wetting behaviors of water droplet onstructured surface and atomic smooth surface based on continuum mechanics, finiteelement method and molecular dynamics.We first studied the static wetting properties of water droplet on nano/microsubstrate experimentally, we found that the contribution of line tension in theboundary of solid-liquid-vapor three phases should not be ignored when the scale ofthe structures were decreased. We construct a new model which can be used topredict the exact value of apparent contact angle. We also found that a critical lengthexist, contact angle will be close to1800when the scale of the structure is less thanthe critical scale. Our theory revealed that decreasing the scale could not onlyincrease the critical pressure, but also decrease the request of the height of the micropillars, so the structures could obtain capacity to avoid be destroyed in destructiveenvironment. We also realize water transportation driven by the gradient of scalealone.We first reported the detailed observation about the sliding behaviors of waterdroplet on hydrophobic micro-pillar structured substrate. When the slope of theinclined plane is close to the sliding angle, the rear contact line will first detach fromthe substrate, and then the front contact line attach to the substrate, the process likethe movement of a caterpillar tractor. According to the law of conservation of energy,we put forward a new model which can be used to predict the sliding angle by threebasic parameters: Young contact angle, the volume of the water droplet and thesolid-liquid area fraction.Base on large scale single crystal graphene, we measure contact angle of water droplet experimentally. Depending on molecular dynamics (MD) and the measuredcontact angle, we calibrate two unknown parameters in L-J potential. What’s more,we revealed the scale effect between contact angle and the radius of carbon nanotube(CNT). We give potential between single particle and infinite substrate theoreticallywhich could be expressed by curvature, and we revealed that curvature can drivesingle molecular move in high-speed.For macroscopical water droplet study, we revealed that the total surfaceenergy is determined by curvature. We realized a kind of high-speed water dropletspontaneous transportation under ambient conditions which is at least two orders ofmagnitude higher than that resulting from any known single spontaneous movementmechanism, for example, Marangoni effect due to gradient of surface tension. Wetrapped even higher spontaneous movement speeds in virtual experiments for dropson nanoscale cones by using MD simulations. We not only give analytical expressionof total surface energy of water droplet on arbitrary curved surface, but also giveanalytical expression of ultimate speed on arbitrary curved surface driven bycurvature.