Laser Processing Of Droplets Pump-free Directional Transport Trajectory And Motion Analysis

Due to the wide application prospect and research value in the fields of biochemical analysis and microfluidic system,the pump-free transportation of droplets on extremely infiltrating surfaces is becoming a hot research topic.In view of the cumbersome preparation method,high cost and poor anti-gravity transporting performance in the research of ultra-pure surface liquid pump-free transportation,this paper uses laser processing method to realize the preparation of a superhydrophobicsuperhydrophilic pumpless transport functional surface based on related theory and experiment.And through the design of the lyophilic zone,the pump-free directional transport speed of the droplets is continuously improved.The main research contents are as follows:(1)A superhydrophobic-superhydrophilic mixed functional surface was prepared by inducing the infiltration change of the surface on an aluminum plate by a pulsed laser.The effect of varied post-treatments on the superhydrophobic surface of the aluminum plate was studied after laser processing of the aluminum plate.The results show that the surface of the aluminum plate can be changed from superhydrophilic to superhydrophobic by heating in air environment or vacuum environment.The contact angle of the sample surface can reach above 160 °.However,the rolling angle of the heated sample in air is more than 30 °,meanwhile the rolling angle of the heated sample under the vacuum atmosphere is only 3.04 °;It is obvious that the superhydrophobic effect of the heated sample in vacuum is better.It could be concluded that the vacuum environment has fewer factors and the sample is more stable.(2)A superhydrophilic-superhydrophobic mixed pump-free directional transport trajectory was prepared on the surface of the aluminum plate by laser method.The effect of the width of hydrophilic stripe on the transport capacity of droplets was studied experimentally.The results show that the droplet can move upward on the slope with a distance of about 1 mm,however this movement is not continuous.The smaller the width of the hydrophilic stripe on the surface of the superhydrophobic-superhydrophilic mixed pump-free transport function,the larger the tilt angle ? when the droplet moves upward.The reason for this phenomenon is capillary force : the smaller the width of the hydrophilic stripes,the larger the droplet capillary force.(3)The wedge-shaped pump-free directional transport trajectory does improve the distance and speed of the droplet's anti-gravity transport from the bottom to the top compared to the stripe-shaped directional transport trajectory.The maximum droplet volume and velocity transported on different wedge trajectories are explored.Experiments show that under the premise of sufficient droplets,the bigger the wedge angle,the larger the droplet volume,the faster the droplet movement speed.When the two different factors of droplet size and channel wedge angle comared,different wedge angles have a greater influence on the droplet transport speed.When the channel wedge angle ? is 10 °,the maximum speed of 3 ?l droplets is about 500 mm/s.And the maximum transport liquid volume of the channel can reach 8.7 ?l.This is because the larger the wedge angle,the greater the difference in surface tension of the droplets.(4)A roughness gradient trajectory with a Ra difference of 10 ?m was prepared by experiment.Experimental results show that the direction of droplet movement is from low roughness to high roughness.Therefore,the displacement of droplets is analyzed.The droplets first move in the “peristaltic” direction on the sample trajectory,and then the motion approaches a uniform velocity.Increasing the number of roughness gradient lines is beneficial to the increment of the maximum volume of pump-free droplets.As far as the overall roughness of the trajectory is concerned,the smaller the overall roughness of the trajectory,the better the pump-free directional transport of droplets.This can be explained as follows: the more the number of tracks,the more unbalanced the force of the droplets,and the greater the difference in surface tension of the droplets.All these research results provide reference value and guidance for further rapid transportation of liquids on curved and non-planar complex trajectories,thus making a better usage in various fields.