Mechanism Of Photoacoustic Synergistic Manipulation Of Micro-nanoparticles

For the past decades,precise manipulation of micro-and nano-particles has become more and more significant in many research fields such as physics,life science and biomedicine.Compared to optical micromanipulation,acoustic micromanipulation has the advantages of low power,nondestructive,high adaptability and evident force effect,and has attracted extensive attention and applications.The core aspect of acoustic micromanipulation is the acoustic radiation force(ARF),which is mainly determined by the acoustic field distribution.Up to now,the theoretical studies of ARF have mainly focused on the particles in various acoustic fields such as plane wave,standing wave,Gaussian beam,focused Gaussian beam and Bessel beam.However,the acoustical micromanipulation using single beam is difficult to satisfy the ever increasing requirements for the multifarious and precise manipulations.In this thesis,we propose a candidate strategy to modulate the ARFs acting on particles using laser irradiation.The acoustic field distribution around the particle can be altered by the laser-induced photoacoustic(PA)radiation from the particle,and then the ARFs acting on particles are modulated effectively.The detailed study is as follows:1.The PA properties of the liquid-immersed gold nanoshells(GNSs)with near-infrared optical responses have been investigated systematically.We study the optical absorption and PA response of the liquid-immersed GNSs using the finite element method(FEM).The influences of the geometry,surrounding medium,laser fluence and laser pulse width on the PA signal of the GNS are discussed in detail.It is found that both the optical absorption and PA signal of the GNS are greatly modulated by the geometry of the GNS.The PA signal intensity can be greatly enhanced by means of the structure optimization.The increased laser fluence leads to the enhanced PA signal.The shorter laser pulse width induces the faster temperature rise and the higher PA conversion efficiency,resulting in the stronger PA signal.Furthermore,we find that the larger Gruneisen parameter of the embedding medium leads to a stronger PA signal2.Modulating the ARF acting on an elastic sphere(ES)in an acoustic plane wave by laser irradiation.We propose to use a sinusoidally modulated laser beam to irradiate the ES in the acoustic plane wave for realizing the effective modulation of ARF.Based on PA effect model and Mie scattering theory,the acoustic scattering of the laser-irradiated ES in an acoustic plane wave is derived mathematically.Then the ARF acting on the laser-irradiated ES is calculated using the Brillouin stress tensor.In addition,we establish the FEM model to mimic the ARF on the laser-irradiated ES in an acoustic plane wave to verify the analytical results.It is found that the PA radiation of the laser-irradiated ES can modulate the ARF effectively.Especially,the negative ARF occurs at certain frequencies,which is ascribed to the competition between the PA radiation and the acoustic scattering of the ES.We further find that there is a region of the phase difference in which the negative ARF can be achieved by changing the laser intensity.If the phase difference is located outside this region,it is then impossible to achieve the negative ARF.The region in which the phase difference for the negative ARF occurs is also dependent on the frequency.3.We further investigate the modulation of ARF acting on the liquid ball in a plane acoustic wave by laser irradiation.The ARFs on a laser-irradiated oleic acid(OA)sphere in a plane acoustic wave are studied by a mathematical model and demonstrated by the FEM simulations.It is found that the ARF on the laser-irradiated OA sphere can be greatly modulated by the laser intensity and the phase difference between the plane acoustic wave and the laser beam.There exists a region of phase difference in which the ARF on the laser-irradiated OA sphere can be changed from positive values to negative values with increasing the laser intensity.At the optimal phase difference,the needed laser intensity for negative ARF is minimal.We further find with increasing the size of the OA sphere that the ARF peaks move to lower frequencies and the amplitudes of the ARF peaks are enhanced.The surrounding medium with higher acoustic impedance enables larger negative ARF.