Research On All Fiber Micromotor Based On Thermal Convection

Fiber optical tweezers(OTs)use optical fibers to converge light beams,generating a stable trapping force that can capture particles.Fiber OTs have broad applications in life science,colloidal physics,and biochemistry.Micromotors based on OTs convert light energy into mechanical energy,and hold exciting prospects on drug delivery,observing fluid microscopic properties,and disease diagnosis.Currently,there are two methods for light-induced rotary micromotors,optical field active driving and external field passive driving.Optical field active driving relies on specially modulated laser beams to transmit angular momentum to particles,or particles have a rotationally symmetric structure that generates torque when interacting with lasers.External field passive driving relies on the thermal field or flow velocity field directly or indirectly excited by the light field to generate torque for particles.The method of actively driving the light field relies on specially modulated light fields or specially designed particles,and the operating systems are complex and require high accuracy.The method of passive driving in the external field requires the design of a system to convert light energy into thermal energy or mechanical energy of the flow field.Currently,the passive driving micromotors require rotating particles to have special properties,or uses complex dynamic scanning light traps.The experimental device structure is complex,so new methods need to be explored to simplify the experimental system and control the rotation of transparent particles.In this thesis,a method based on fiber OTs to stimulate thermal convection rotating micromotor was proposed,and this method utilized the principle of passive external driving of micromotors.The experimental system had a simple structure and is easy to operate.Using a special current discharge machining process,a parabolic optical fiber probe emitting Bessellike beams was designed and fabricated.The probe was coaxially fused to single mode and multimode fibers,and part of the parabolic region was coated with a gold film to stimulate thermal convection.A portion of the laser beam converged through the parabolic end surface to generate optical trapping forces for stably capturing cells.Another portion of the radiation generated heat on the gold film,causing the water temperature near the gold film to rise.Water with high temperature has low density and moves upward under the action of buoyancy.On the contrary,water with low temperature has high density and moves downward under the action of gravity,resulting in a convection.A non spherically symmetric cell trapped by fiber optical tweezers is subjected to drag forces in the water velocity field with a velocity difference,generating a torque that drives the cell to continuously rotate.This thesis used simulation software to explore the factors that affect the light field emitted by the probe,optimized the structural parameters of the probe using thermal and flow field simulation results,simulated the output light field of the fiber optic probe and the excited thermal flow field according to the optimized probe parameters,calculated the temperature rise of water under different laser powers and the velocity gradient of water flow near the probe,and calculated the theoretical value of particle rotation rate under different laser powers.At the same time,a fiber probe was fabricated based on the structural parameters determined by simulations,an experimental system was built,and micromotor experiments were completed.The experimental results show that when the incident laser power is higher,the microparticle rotates faster.The rotation rate varies from 12 to 80 rpm.This rotary micromotor has potential applications in microfluidic pumping,biopsy,micromanipulation,etc.