Fabrication Of Micro-nano Structure On Copper Surface With Special Wettability And Application In Fluid Flow

Lots of attentions have been paid to fog collection in recent years due to the growing fresh water crisis arising from expansion of arid region and pollution of water resource.The reported studies,however,are mostly based on the abundant fog flow rather than the ordinary circumstances.The detailed influence of different surface nanostructures on fog collection,especially on fog capture and droplet motion,still remains uncertainty.On the other hand,capillary imbibition phenomenon has attracted considerable attention recently because of its widely application in heat and mass transfer,aerospace,lab-on-chip,diagnostic facilities.However,the relevant experimental verification and theoretical prediction of capillary flow enhanced by surface of gradient wettability in a channel have not yet been reported,to the best of our knowledge.Studies on the acceleration of capillary motion by constructing segmented wettability surface（e.g.,gradient wettability on the first segment of the channel and homogeneous wettability on the second segment of the channel）have also been rarely reported.The conical copper wires were subjected to alkali assistant oxidation or electrochemical deposition to form superhydrophilic wettability of～4^o（water contact angle）with needle-like or leaf-like morphology,respectively.The superhydrophobic conical copper wires with water contact angles of～156^o were further constructed by modification with 1-dodecanethiol.From XRD patterns,the peaks were indexed to Cu（OH）₂ on the surface fabricated by alkali assistant oxidation and Cu on the surface constructed by electrochemical deposition.The conical copper wires were used to study the effect of fog flow velocity and surface morphology on fog collection.With the fog flow velocity increased,the dominant factor for fog collection changed from fog capture to droplet motion.The superhydrophilic surfaces with needle-like morphology displayed better fog capture ability while the superhydrophobic surfaces with leaf-like morphology were more prone to driving droplet motion.The open semicircular channels were subjected to alkali assistant oxidation to construct inner capillary surface wettability/outer plane surface wettability of 3°/3°,87°/87°,3°/87°,87°～3°/3°respectively.The channel with wettability of 3°/87°needed the minimum volume of droplet to slide.The channel with gradient wettability of 87°～3°on the inner capillary surface,influenced a little bit on the droplet volume in droplet slide experiment,but made the droplet diffusion distance decrease by 1.1%～16.3%and velocity increase by 48.5%～52.5%.Open semicircular copper channels with gradient wettability of 1,2-propanediol contact angle 45o～3oor 3o～45o,homogeneous wettability of 3o,and segmented wettability（with gradient segment and homogeneous segment）on their surfaces were constructed by alkali assistant oxidation,respectively.The capillary imbibition of 1,2-propanediol in these horizontally mounted channels demonstrates that compared with the surface with homogeneous wettability of 3^o,there are additional driving force and resistance on the surfaces with gradient wettability of 45o～3oand 3o～45orespectively and thus they lead to the deceleration ranging from 25.0%to 28.6%and the acceleration ranging from 11.1%to 30.0%respectively.In the same length and radius,the channel with gradient wettability of 45o～3oon the first half segment and homogeneous wettability of 3oon the second half segment could accelerate capillary imbibition better than that with gradient wettability of 45o～3oon the whole channel.By constructing the positive gradient wettability only on the inner surface of the first segment in the channel instead of the whole channel,the gradient force per unit length can be increased and additional force can be brought to the inner surface of the second segment of the channel.Furthermore,smaller length ratio（length of gradient segment to that of homogeneous segment）of the channel demonstrates better liquid acceleration effect.Based on the Lucas-Washburn equation,a theoretical model to describe the capillary imbibition was established by introducing an additional force arisen from the gradient wettability and a retardation coefficient caused by the surface roughness.The flow front position versus flow time could be predicted by the established model in this work,and the relative errors between the theoretical prediction and experimental result were less than 20.7%,indicating rationality of the proposed model.Open semicircular copper channels with gradient wettability of water contact angle45o～3oor 3o～45o,homogeneous wettability of 3oon their surfaces were constructed by alkali assistant oxidation,respectively.The capillary rise of water in these inclined channels demonstrates that,compared with the surface with homogeneous wettability of 3^o,there are additional resistance on the surfaces with gradient wettability of 3o～45o,thus they lead to0.006～0.0073 m equilibrium height and deceleration ranging from 2.0%to 18.2%.However,additional driving force was found on the surfaces with gradient wettability of 45o～3o,leading to the acceleration ranging from 18.2%to 43.0%.The gravity terms were introduced into the theoretical model and the errors between theoretical prediction and experimental value were less than 8.7%.Open semicircular copper channels with shape gradient of 0.5～0.75×10^-3 m,0.5～1×10^-3m,0.5～1.5×10^-3 m,0.75～0.5×10^-3 m,1～0.5×10^-3 m,1.5～0.5×10^-3 m were constructed.The wettability of the inner surfaces of these channels were all constructed as homogeneous 3°（water contact angle）.The capillary rise of water in these inclined channels demonstrates that,channels with large radius scope per unit length made liquid move faster.Under the same radius scope per unit length,the channels with radius from wide to narrow imbibe liquid faster than that with radius from narrow to wide.A linear relationship between the radius and the flow front position was introduced to the theoretical model for the channel with a shape gradient.The errors between theoretical prediction and experimental results were within 8.5%.Furthermore,a composite gradient channel with a radius of 0.5～0.25×10^-3 m and a water contact angle of 45°～3°was prepared for the capillary rise experiment.The velocity of liquid moving in composite gradient channel is faster than that of single wettability gradient or single shape gradient channel by 20.6%～68.1%.The wettability gradient plays a more important role in the composite gradient than the shape gradient.The theoretical model combines wettability gradient and shape gradient.The errors between theoretical prediction and experimental results were less than 19.2%.