| Microfibers are the key components for manufacturing fiber based reinforcement materials,bionic materials,fiber based microstructures,and fiber based micro-nano electromechanical systems.Their assembly accuracy and distribution mode directly affect the performance of products.However,due to the scale effect,the force related to volume is no longer the dominant force in the microscale,but the force related to area,which makes it difficult to pick-up and release microfibers accurately.At present,the common microfiber operation methods mainly focus on the overall directional layout of multiple microfibers,and there are still challenges in the precise picking,release and layout of a single microfiber.Therefore,this thesis proposes a high-precision micromanipulation and controllable layout method for a single microfiber.The specific research contents are as follows:(1)The key conditions of microfiber picking,releasing and self-alignment are obtained by analyzing the force acting on the microfiber operation process:the first strategy picks-up the microfiber under the condition that the picking force is greater than the sum of the adhesion force between the microfiber and the substrate and its own gravity.The condition of the second strategy is that the picking force is greater than the sum of the releasing force and the microfiber gravity.The microfiber release condition is that the sum of release force and microfiber gravity is greater than the picking force.The self-alignment of the microfiber needs to limit enough liquid drop in the groove,so that the liquid bridge between the microfiber and the groove presents a convex liquid surface.The picking force and releasing force are solved by double iteration algorithm,and the influencing factors and dominant relationship between them are explored.The results show that the picking force increases with the increase of contact length and the decrease of needle contact angle,and the release force increases with the decrease of substrate contact angle and the increase of liquid bridge volume.The dominant relationship between the two is mainly affected by the contact angle of the substrate and the volume of the liquid bridge.When the contact angle of the substrate is 60°～80°,the release force will gradually dominate with the increase of the volume of the liquid bridge.The influence of liquid bridge volume on microfiber self-alignment is studied through simulation,and the minimum theoretical liquid bridge volume required for microfiber self-alignment with grooves of different widths is obtained.The relationship between angle error,linear error and energy,restoring torque and restoring force in the process of selfalignment is analyzed.The results show that the energy,restoring torque and restoring force decrease with the decrease of angle error and linear error.(2)The liquid bridge fracture in microfiber operation was explored through simulation,and the liquid bridge fracture conditions for microfiber pickup and release were obtained.The effects of wettability difference value,needle speed and liquid bridge volume on the ratio of liquid bridge fracture height and liquid drop transfer rate were studied.The results show that the ratio of liquid bridge fracture height and droplet transfer rate decreases with the increase of wettability difference value.When the wettability difference value is negative,the ratio of liquid bridge fracture height and droplet transfer rate are usually above 50%,and the microfiber is easier to be released.On the contrary,when both are below 50%,micro fiber is easier to be picked-up.The influence of needle speed and liquid bridge volume on liquid bridge fracture is mainly reflected in the less hydrophilic substrate.The smaller the needle speed,the larger the volume of the liquid bridge,the higher the liquid bridge fracture height ratio and the droplet transfer rate,the more difficult to pick-up the microfiber,and the easier it is to release.(3)An automatic microfiber operation platform for microfiber picking,transfer,release,self-alignment and patterned layout has been built.The wettability adjustable substrate for microfiber manipulation was prepared by laser micromachining technology.The effects of scanning line spacing,scanning speed and defocusing distance on substrate wettability were studied.The results show that the substrate wettability can be controlled in the range of 5°～130°,and the substrate contact angle increases with the increase of scanning line spacing,scanning speed and defocusing distance.The superhydrophilic-superhydrophobic groove structure for microfiber self-alignment was prepared,and the relationship between processing parameters and groove size was analyzed.The results showed that the groove width and groove depth increased with the increase of scanning lines and scanning times,respectively.The droplet limitation experiment was further carried out for the groove structure.The results show that the volume of liquid drops confined by the groove structure is much larger than the minimum theoretical liquid bridge volume required for microfiber self-alignment.(4)A series of microfiber operation experiments were carried out on the microfiber operation platform.First,the initial droplet boundary of the two picking strategies is obtained,and the microfiber picking is realized on the superhydrophilic surface.Then,the effects of wettability difference value,needle speed and liquid bridge volume on the ratio of liquid bridge fracture height,liquid drop transfer rate and success rate in microfiber operation were studied through experiments.The results showed that with the increase of wettability difference value,the ratio of liquid bridge fracture height and liquid drop transfer rate decreased,the success rate of microfiber pick-up increased,and the success rate of release decreased.The influence of needle speed and liquid bridge volume on microfiber operation is mainly shown in the area of 0°≤C≤40°.With the slowing of needle speed and the increase of liquid bridge volume,the success rate of microfiber pick-up decreases and the success rate of microfiber release increases.Secondly,the influence of groove width,angle error,linear error and pick position on the success rate of self-alignment is studied through self-alignment experiments.The results show that the success rate of self-alignment under different errors can reach 100%.The relationship between groove size and self-alignment accuracy is analyzed.The results show that the linear alignment accuracy can reach 0.33 μm.The angular alignment accuracy can reach 0.39°at most,both of which increase with the decrease of the groove width.At last,the patterned layout of microfibers is realized,and the operation speed is evaluated.The results show that the microfiber operation method proposed in this paper can complete the operation of about 3600 micro fibers in 1 hour.In summary,the microfiber operation method proposed in this thesis can achieve fast and accurate picking,transfer,release,self-alignment and patterned layout of a single micro fiber in a flexible contact way,with high reliability.It solves the problems of precise release in microfiber operation and precise control of microfibers spacing and layout form,providing a new idea for microfiber operation.This method has potential application value in preparing highperformance fiber based materials,microstructures and MEMS. |
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