The Study Of Optically Controlled Macroscopic Rotation Of Ferrofluid Under Magnetic Field

Since the inception of optical tweezers technology,it has garnered considerable attention from numerous researchers due to its outstanding manipulative abilities in the microscopic realm.Consequently,this technology has played a pivotal role in the fields of biomedicine,physics,and chemistry.In addition to optical tweezers,researchers have also harnessed the thermal effects of light to capture and propel microscopic particles.In the macroscopic domain,the manipulation of object motion using light has likewise piqued researchers’ curiosity,with numerous intricate light-controlled ma.croscopic object movements already achieved,such as light-induced solid deformation,locomotion,and rotation,as well as liquid deformation and movement.However,utilizing light to manipulate macroscopic liquid rotation remains an arduous challenge due to the inherent contradiction between the asymmetry of liquid rotational motion and the symmetry of optically-induced thermal convection.To address this issue,the present study proposes a principle involving the use of a ferrofluid—a unique magnetically responsive liquid material—and successfully achieves light-controlled liquid rotation through the coupling of light,magnetic,and fluidic fields.The contents of this thesis can be divided into two sections:(1)Initially,theoretical explorations were conducted regarding the magnetization characteristics,rheological properties and mechanical attributes,such as superparamagnetism,magnetoviscous effect and Kelvin body forces,of ferrofluid in magnetic fields.Furthermore,the working mechanisms of thermal convection modes in ferrofluid,like natural convection,Marangoni convection and thermomagnetic convection,were studied in detail.Experimental verifications on the distribution characteristics of thermomagnetic convection and Marangoni convection were also performed.Based on this,a temperaturesensitive ferrofluid equilibrium system was established by utilizing the differences in temperature responses between the magnetization effect and gravity.By controlling laser radiation positions,the non-equilibrium forces in the ferrofluid were transformed into asymmetric factors that could disrupt the symmetry of convection,which successfully achieved optically controlled rotational motion of the ferrofluid under magnetic fields.The impacts of different factors on the rotation of ferrofluid,such as laser power,magnetic field intensity,tilt angle and laser radiation position,were also explored.In addition,in-depth studies on the conditions and principles of rotation generation were conducted through experiments to provide reasonable explanations for the optically controlled rotation phenomenon of ferrofluid.(2)By coupling the static magnetic field equation,Navier-Stokes equation under the laminar flow condition,Kelvin volumetric force,and Langevin magnetization model of ferrofluids,a computational fluid dynamics model for light-controlled ferrofluid motion was derived.Based on this,two key experiments were selected for simulation validation,with the geometric structure and boundary conditions of the model established according to experimental conditions.Employing the robust multiphysics coupling and finite element computation capabilities of COMSOL Multiphysics,simulation results of temperature and flow field distributions were obtained.The distribution trends derived from calculations were consistent with experimental results,providing theoretical validation for the mechanism of ferrofluid rotational motion.The realization of light-controlled ferrofluid rotation further broadens the capabilities and application scope of light-controlled material manipulation,injecting new vitality into research and application in related fields such as liquid robots,microfluidics,and object transportation.