Experimental Studies on Extremely Small Scale Vibrations of Micro-Scale Mechanical and Biological Structures

Abstract

Experimental vibration analysis of mechanical structures is a well established field.Plenty of literature exists on macro scale structures in the fields of civil, mechanical and aerospace engineering, but the study of vibrations of micro scale structures such as MEMS, liquid droplets, and biological cells is relatively new. For such structures, the amplitudes of vibration are typically in nanometeror sub-nanometer range and the frequencies are in KHz to MHz range depending upon the dimensions of the structure. In our study, we use a scanningLaser Doppler Vibrometer (LDV) to measure the vibrations of micro-scale objects such as MEMS structures, micro droplets and cells. The vibrometercan capture frequency response up to 24 MHz withpicometer displacement resolution. First, we present the study of dynamics of a 2-D micromechanical structure—a MEMSelectrothermal actuator. The structure is realized using SOI MUMPs process from MEMSCAP. The fabricated device is tested for its dynamic performance characteristics using the LDV. In our experiments, we could capture up to 50 out-of-plane modes of vibration—an unprecedented capture—with a single excitation. Subsequent FEM based numerical simulations confirmed that the captured modes were indeed what the experiments indicated, and the measured frequencies werefound to be within 5% of theoretically predicted. Next, we study the dynamics of a 3-D micro droplet and show how the substrate adhesion modulates the natural frequency of the droplet. Adhesion properties of droplets are decided by the degree of wettability that is generally measured by the contact angle between the substrate and the droplet. In this work, we were able to capture 14 modes of vibration of a mercury droplet on different substrates and measure the correspondingfrequencies experimentally. We verify these frequencies with analytical calculations and find that all the measured frequencies are within 6% of theoretically predicted values. We also show that considering any two pairs of natural frequencies, we can calculate the surface tension and the contact angle, thus providing a new method for measuring adhesion of a droplet on an unknown surface. Lastly, we present a study of vibrations of biological cells.Our first study is that of single muscle fibers taken from drosophila.Muscle fibers with different pathological conditions were held in two structural configurations—asa fixed-fixed beam and a cantilever beam—and their vibration signatures analysed.We found that there was significant reduction in natural frequency of diseased fibers. Among the diseased fibers, we could confidently classify the myopathies into nemaline and cardiac types based on the natural frequency of single fibers. We have noticed that the elastic modulus of the muscle which decides the natural frequency is dictated by the myosin expression levels. Our last example isa study of the vibration signatures of cancer cells. Here we measure the natural frequencies of normal and certain cancerous cells, and show that we can distinguish the two based on their natural frequencies. We find that the natural frequency of cancerous cells is approximately half of that of normal cells. Within the cancerous cells, we are able to distinguished epithelial cancer cells and mesenchymal cancer cells based on their natural frequency values. For Epithelial cells,we activate the signaling pathways to induce EMT and notice the reduction in the natural frequency. This mechanical assay based on vibration response corroborates results from the biochemical assays such as Western blots and PCR, thus opening a new technique of mechano-diagnostics.