초음파를 이용한 미세입자의 포획 및 조정

Abstract

Recently, the manipulation of very small objects is necessary in the fields of nano-technology, microbiology, biochemistry, etc. In many cases, such particles are too small to move to a particular position in a medium by using a general mechanical tweezer technology. The optical tweezer using LASER has been found to have many applications in the trapping of macroparticles including biological molecules and cells. However, LASER has some weak points that the light is hard to transmit in opaque media and its high power could affect the targets, especially biological ones. To avoid those problems, ultrasound has been adapted to microparticle trapping and manipulation. The acoustic radiation force which is the force exerted on a particle in a focused ultrasonic fields can be calculated theoretically by using a ray acoustics theory. Also it is possible to calculate kinetic energy density and potential energy density of standing waves. According to the calculation results, the acoustic radiation force can be derived. Therefor, the force in an ultrasonic standing wave field is well known by many researchers. However, little experimental research has been reported on those for the cylindrical standing wave field and a strongly focused field. In this study, in order to figure out the positions where microparticles are trapped in the standing wave field, especially a cylindrical standing wave field, we derived the equations giving the radiation force and the potential energy distribution. Then, the trapped pattern and their variation with time in a cylindrical transducer were simulated by COMSOL, a finite element method. The simulation results show that the polystyrene particles are moved to and aggregated near the positions corresponding to pressure nodes which were estimated from the derived equations. Those phenomena were observed and confirmed by measurement. The measurement results show that micro sphere particles were trapped well at pressure nodes of the standing waves and their position can be easily manipulated by frequency control. In addition it is demonstrated that the biological particles of green algae, Chlorella, shows the similar trapping phenomena as the polystyrene particles. The trapping results by the focused beam show that the radiation force by single focused beam pushes a particle away from the center of transducer and two focused confronted beams can trap it at near center.