Undirectional Wetting Mechanism Of Droplet On Wettability-Patterned Gradient Surface And Related Application On Microfluidic Device

In nature,the surfaces of biologies present patterned morphologies or structures.Different sizes,spacings and configurations of the patterned surfaces lead to various kinds of wettability,such as superhydrophobicity,superhydrophilicity,anisotropy,unidirectional wettability and so on.In recent years,inspired by the surface array structures and their corresponding wetting characteristics of the biologies,the artificial patterned array structures also present diversity and are widely used in many fields,such as self cleaning,fluid control,water collection and microfluidics by means of the advanced bionic engineering.However,some of the internal mechanisms are still unclear.In this paper,from the perspective of self-driven mechanism and application,taking wettability-patterned gradient surfaces into account,the wetting states,the motion behaviors of droplet and related application on open microfluidic device have been investigated theoretically and experimentally.The main research works are as follows:(1)The analytical model to establish conditions for the incipient spontaneous motion of droplets with different surface tensions on wettability-controlled gradient tracks are proposed.We design five types of wettability gradient surfaces with circular-shaped superhydrophilic patterns and predict the spreading distances of the droplets with different surface tensions on these surface through theoretical calculation.Furthermore,the microfluidic devices with corresponding wettability gradients are achieved to examine the accuracy of the predictions.Finally,the microfluidic devices can also be used for the facile determination of medical alcohol composition.As an application,we develop the so-call medicinal alcohol meter that can readily determine if the concentrations of the medicinal alcohol for physical defer-vescence and bed-sore prevention are still lying in the valid range.(2)The influence of the shape changes of superhydrophilic units on the anisotropic wetting is theoretically analyzed.From the perspective of driving force,we explan that why the unidirectional propagation distance of droplets on the anisotropic patterned gradient surface is larger than that on the isotropic patterned gradient surface,and it is verified by experiments.A method of combining the gradient of shape with patterned gradient is put forward.Experimental results show that the propagation distance of droplet on compound gradient surface is longer than that on strip-patterned gradient surface.(3)Taking the asymmetric slanted micro-pillar arrays as the research object,the wetting state of the droplet on the slanted micro-pillar patterned surface is theoretically analyzed.Secondly,the criterion of unidirectional transportation of droplet on the surface is studied,and the influence of structural size and inclination degree on it is also discussed.The advancing angle and the retreating angle along the inclined direction and the reverse direction of the droplet on the slanted micro-pillar patterned surface are analyzed respectively,and the reason why the droplet will take precedence in the direction of the inclined direction is explained.Finally,the slanted micro-pillar patterned surface is prepared by experiment and the unidirectional propagation velocity and distance are improved by using a variety of chemical modification methods.(4)Wettability-patterned gradient integrated with wedge-shaped track are applied to paper-based microfluidic chip,which increases the wetting difference inside and outside of the track,prevents droplets from spreading out of the channel and solves the problem of insufficient accuracy.Then,the influence of the variation of the wedge angle and the wettability-patterned gradient and geometrical configuration of the track on droplet transport are both analyzed and studied.Paper-based microfluidic chips based on wedge-shaped track with wettability-patterned gradient are applied to detect the concentrations of glucose solutions,which can be helpful to the early diagnosis,prevention and treatment of diabetic patients.The micro-mixers without pump are also designed and prepared.The corresponding experiments show that the increase of the wedge angle and wettability-patterned gradient can effectively improve the mixing rate of micro-mixer.In summary,the gradient-patterned surafce has a very important influence on the wettability of solid surface,which has good applications in microfluidics,rapid medical diagnosis,environmental detection,food safety detection and other fields.