Chemical Micro-Patterned Surfaces On Droplet Evaporation

Droplet evaporation is widely used in cooling integrated circuits,inkjet printing,DNA mapping and other fields.Generally,the properties of the material surface have a crucial effect on the evaporation of the droplets.However,the current researches on droplet evaporation mostly focuses on homogeneous substrates,there are few researches on heterogeneous pattern surfaces.Therefore,photo catalytic activity of titanium dioxide combined with UV photolithography technology were used to prepare a variety of patterned heterogeneous surface on titanium in this paper,the evaporation properties of the sessile droplets on these surfaces were investigated.Firstly,the anodic oxidation method was used to prepare TiO2 nanotubes with photo catalytic activity on the surface of pure titanium.At the same time,nanoscale roughness was provided for the surface.After treatment by the hydrophobic agent,the titanium was irradiated with ultraviolet light through the mask,the photo degradation of the hydrophobic groups in the light-transmitting portion were converted to hydrophilic groups.The titanium heterogeneous surface forms superhydrophobic/hydrophilic pattern while the contact angle of hydrophobic parts is up to 167° and hydrophilic parts is under 10°.The surface had a good ability of controlling the droplet distribution to form a clear pattern of water.Secondly,studying the effect of surface wettability differences on droplet evaporation,the evaporation behavior of single droplet on homogeneous surface and patterned heterogeneous surface was compared.The results showed that there are two phases in the evaporating process of the droplets on the surface of the heterogeneous pattern-the hopping mode evaporation phase and the stick-slip mode phase.The constant contact line(CCL)mode evaporation period take up to 70% of the total evaporation time on homogeneous surface,while the increase of the contact angle,the droplet evaporation time prolongs.The CCL mode evaporation phase is about 80% of the total evaporation time on patterned heterogeneous surface.It is different that the droplet evaporation jump and stick-slip mode in different patterns,the overall performance of the longer the pinning,jump less the number of droplet evaporation shorter time-consuming.Generally,the longer the pinning time,the less the number of jumps,the shorter the evaporation time of the droplet.The evaporation characteristics of droplets on the micro-patterned heterogeneous surface were studied.The effects of different patterns,hydrophilic/hydrophobic ratio and wall temperature on the droplet evaporation were discussed.Droplets evaporated more frequently on the heterogeneous surface of the reticulated micro-patterned and take longer time to evaporate.In the checkered island micro-patterned,the evaporation mode was mostly focuses on CCL mode and the evaporating time is shorter.An increase about hydrophilic ratio will increase diameter of the contact surface and promote evaporation.Compared to a 1:3 hydrophilic/hydrophobic surface,the surface evaporation time of 2:1 decreased from 695 s to 615 s.Sessile droplets were mainly affected by the wall heating temperature on the heating surface,the higher the temperature the faster the evaporation.Increased temperature by 20°C(from 40°C to 60°C)reduces the evaporation time of liquid droplets by 66.9%.Finally,the irregular shrinkage of slender liquid droplets on stripe micro-patterned heterogeneous surfaces was studied.It was showed that the relative ratio is an important parameter to droplet shape.The lower the relative ratio is,the more slender the droplet is.When the relative ratio is greater than 5,the droplet is approximately spherical.When the droplets evaporated on the surface of the striped micro-pattern,shrinkage occurred in the parallel direction first,and then the stick-slip phenomenon occurred in the vertical direction.In general,the slender droplets evaporated faster than the spherical droplets,mainly because of the aspect ratio played a role in the evaporation and diffusion of droplets.When the relative fringe ratio is increased from 0.2 to 5,the droplet aspect ratio decreases from 3.2 to 1.02,and the evaporation time decreases by 37.1%.