We show how attractive interactions dramatically influence emulsion rheology. Unlike the repulsive case, attractive emulsions below random close packing, φ(RCP), can form soft gel-like elastic solids. However, above φ(RCP), attractive and repulsive emulsions have similar elasticities. Such compressed attractive emulsions undergo an additional shear-driven relaxation process during yielding. Our results suggest that attractive emulsions begin to yield at weak points through the breakage of bonds, and, above φ(RCP), also undergo droplet configurational rearrangements.
Understanding and controlling modal coupling in micro/nanomechanical devices is integral to the design of high-accuracy timing references and inertial sensors. However, insight into specific physical mechanisms underlying modal coupling, and the ability to tune such interactions is limited. Here, we demonstrate that tuneable mode coupling can be achieved in capacitive microelectromechanical devices with dynamic electrostatic fields enabling strong coupling between otherwise uncoupled modes. A vacuum-sealed microelectromechanical silicon ring resonator is employed in this work, with relevance to the gyroscopic lateral modes of vibration. It is shown that a parametric pumping scheme can be implemented through capacitive electrodes surrounding the device that allows for the mode coupling strength to be dynamically tuned, as well as allowing greater flexibility in the control of the coupling stiffness. Electrostatic pump based sideband coupling is demonstrated, and compared to conventional strain-mediated sideband operations. Electrostatic coupling is shown to be very efficient, enabling strong, tunable dynamical coupling.
This paper presents the use of our approach to comprehensive measurements of the quality factor ( Q) of 1-MHz microelectromechanical system (MEMS) tuning fork resonators. We examined the most important mechanisms that are believed to limit the quality factor in MEMS tuning fork resonators (i.e., gas damping, thermoelastic dissipation (TED), anchor damping, and Akhiezer damping), and we were able to quantitatively account for each mechanism and to eliminate several from consideration. We take advantage of the elimination of TED at ∼120 K, where the linear coefficient of thermal expansion (CTE) becomes 0. These observations enabled the first direct examination of the strength of anchor damping in megahertz tuning fork resonators, allowing the study of the effect of anchor design and other factors. In this megahertz frequency range, the wavelength of elastic waves far exceeds the dimensions of the die, so commonly used models cannot make predictions of anchor damping. Our results show that elastic energy can escape from the resonator through the anchor(s) and still be retained within the die. We find that anchor damping in these megahertz resonators is impacted more by die attach structures at the boundaries of the die than by the resonator anchor designs within the die.[2018-0038] Index Terms-Quality factor, cryogenic experiments, thermoelastic dissipation, coefficient of thermal expansion, anchor damping.
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