The literature describes research about particle motion and manipulation that mainly relies on Atomic Force Microscope (AFM)-based techniques. However, a major concern with this approach is the physical contact requirement with the particles to be moved. Such contact causes serious practical problems. One alternative mechanism, proposed by Professor Cetinkaya’s research group, is to excite the motion of particles by means of base excitation using high-frequency acoustic fields. Their recent work has generated a class of particle pack motion on dry surfaces. As the substrate is excited by an acoustic field consisting of short pulses with variable pulse repetitive rates, translating particles move along curved trajectories on the surface. Surface motion with nanometer level amplitudes is due to a combination of axial and radial vibrational components of the substrate. Because of the complex nature of the exciting base motion field, particle packs could make oscillatory motions on the surface. However, it is observed that, instead of forming a stiff bond with the stationary microsphere and becoming immobile, the free microspheres rotate around the stationary particles and often make a single frequency rotational motion. The stability of these pack configurations is attributed to the rolling moment resistance between the particles in contact. Professor Cetinkaya has developed a non-contact adhesion measurement technique based on the acoustic base-excited rocking motion of microspheres. Transient out-of plane responses of 21 μm polystyrene latex particles on the excited substrate are measured using an interferometer. The resonance frequency of the rotational motion is related to the work of adhesion of the particle and substrate materials by utilizing the rolling resistance of the particles. This study has also experimentally demonstrated for the first time the existence of rolling moment resistance of an adhesion bond between a microsphere and a flat surface.
Figure 2: SEM image of a rotational disk element fabricated for mass detection MEMS sensors.
In addition to investigating the motion, adhesion, and removal of nanoparticles, Professor Cetinkaya’s Industrial Process Monitoring Group focuses on the noninvasive/non-contact characterization and monitoring of industrial processing using acoustic waves. This Group uses acoustic waves to monitor operations involved in the pharmaceutical and semiconductor industries. In addition to classical ultrasonic elastic wave generation and detection, his group utilizes a pulsed laser and air-coupled transducers to generate acoustic waves in structures (without contact). Then they detect the response of these structures by using interferometers and signal processing tools.
Figure 3. Instrumentation diagram of the acoustic system developed for monitoring sputtering targets used in PVD tools.