Ultrasound Powered Rotation And Self-assembly Of Metallic Microparticles

Self-powered micromotors absorbed energy from their surroundings to initiate and maintain their motions.They had many potential applications,including the drug delivery,environmental control and monitoring,medical analysis and treatment,and etc.In particular,ultrasound-powered micromotors utilized the ultrasonic energy in the megahertz frequency range,which is compatible to regular human bio-functional activities.Ultrasound-powered microrods could move at a speed covering a distance hundreds times of their lengths in one second,and their motions and assembly patterns could be controlled by the ultrasonic frequency and magnitude.These micromotors,after levitated in water by ultrasonic waves,could move randomly,spin with respect to the axis along the rod,or rotate with respect to some axes that are aside and perpendicular to the rod.The objective of this thesis was to probe the underlying mechanisms of these ultrasound-powered motions,such that accurate and precise controls of these motions could be applied.In this thesis,we first synthesized gold microrods,300 nm in diameters and 2 ?m in length,by electrodeposition using the alumina templates.Experiments suggests that the linear speeds and angular speeds of microrod that orbit in tight circles scale to V2.In addition,microrods rotated faster when the frequency of ultrasound was closer to the resonance frequency.When the ultrasound frequency shifted away from the resonance frequency,the trajectories of orbiting rods changed from circles to more linear or random shapes.We also studied the behaviors of micromotors that spin around their long axis.We prepared Si O2-Ti Janus microspheres,and deduced their angular speeds by tracking their optical contrast.It was found that with the increase of the ultrasonic driving voltage,the angular speed of the sphere was increased and was approximately quadratic with respect to the ultrasonic driving voltage.As the ultrasonic driving frequency was adjusted to be closer to the resonant frequency,the sphere rotated faster.We studied the effect of hydrophobic modification on the ultrasound powered micromotors.Experiments suggest that the speed of the gold rod which had been modified into hydrophobic was higher than unmodified gold rods.We also found that ultrasound powered microrods assembled into chains at specific ultrasound frequencies.By increasing the ionic strength of the solution,the assembled chain was more stable,maintaining the structure even if ultrasound was repeatedly switched on and off.To summarize,we have analyzed the behaviors of ultrasound powered micromotors,and explained their motion mechanisms.Through hydrophobic modification,the motor speed was improved,and we have found a method to assemble microrods into a stable one-dimensional chain.Overall,this thesis could pave the way for developing more controllable micromachines in biomedical applications.