Antibubbles enable tunable payload release with low-intensity ultrasound

The benefits of ultrasound are its ease-of-use and its ability to precisely deliver energy in opaque and complex media. However, most materials responsive to ultrasound show a weak response, requiring the use of high powers, which are associated with undesirable streaming, cavitation, or temperature rise. These effects hinder response control and may even cause damage to the medium where the ultrasound is applied. Moreover, materials that are currently in use rely on all-or-nothing effects, limiting the ability to fine-tune the response of the material on the fly. For these reasons, there is a need for materials that can respond to low intensity ultrasound with programmable responses. Here it is demonstrated that antibubbles are a low-intensity-ultrasound-responsive material system that can controllably release a payload using acoustic pressures in the kPa range. Varying their size and composition tunes the release pressure, and the response can be switched between a single release and stepwise release across multiple ultrasound pulses. Observations using confocal and high-speed microscopy revealed different ways that can lead to release. These findings lay the groundwork to design antibubbles that controllably respond to low-intensity ultrasound, opening a wide range of applications ranging from ultrasound-responsive material systems to carriers for targeted delivery.Comment: Main Text: 14 pages, 4 figures. Embedded SI: 4 pages, 5 figure

Optical breakdown acoustics : transduction and sensing underwater

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 191-199).In the sea, infrastructures such as ships, pipelines, and wind turbines are exposed to harsh conditions that can wear down the structures through wave loading and corrosion. Because of these wear mechanisms, maritime structures require regular inspections to identify early signs of damage or fatigue. Currently, inspections are performed visually or with contact acoustic transducers, often by a human diver. However, these methods are slow and costly, and can be hindered by surface irregularities like biofouling. Therefore, new sensing techniques are needed to meet the rising demand for offshore infrastructure monitoring. In this thesis, I develop optical breakdown as an acoustic source for non-contact measurements of underwater structures. Optical breakdown occurs when a high-power laser is focused to a small spot, causing nonlinear interactions between the light and water. A compact plasma forms at the focus and expands explosively, radiating a loud, broadband pressure wave.Since this source is compact, laser-controlled and broadband, it provides unique sensing capabilities that overcome challenges faced by traditional transducers. First, I demonstrate how the breakdown source can be used to remotely measure the internal properties of submerged plates. The source is used to excite leaky Lamb waves in the plates, and broadband elastic dispersion spectra are measured using hydrophones in the water. The dispersion spectra are used to calculate the thicknesses and sound speeds in aluminum, steel, bronze and glass plates of varying thickness. Second, I characterize how the source can be controlled and scaled up by combining acoustic measurements with high-speed images of the breakdown plasma. In general, breakdown produces a loud (>100kPa at 10cm), ultra-broadband (5kHz-5MHz) source, whose characteristics depend on measurement orientation and laser properties.This transduction behavior is explained by modeling the breakdown plasma as an array of laser-driven explosions. When the laser is tightly focused, the plasma is compact, producing a loud and omnidirectional signal. However, for weak focusing and high energies, the plasma lengthens and becomes erratic, producing a weaker signal with less consistent behavior. These results reveal design challenges, tradeoffs and opportunities when adapting the breakdown source for dierent applications.by Athanasios G. Athanassiadis.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin

Parallel pulsed jets for precise underwater propulsion

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 95-98).A significant limitation for underwater robots is their ability to maneuver in tight spaces or for complex tracking tasks. Next generation vehicles require thrusters that can overcome this problem and efficiently provide high maneuverability at low speeds. Recently, thruster design has begun to draw inspiration from nature's swimmers, applying the principles of pulsed jet propulsion to robotic thrusters. Although most developments have focused on single jet actuators, nature provides some indications that multi-jet systems can provide propulsive benefits -- marine invertebrates called sales connect into chains of individual animals that each eject short jets to collaboratively move the entire chain efficiently around the ocean. However, despite the promise of multi-jet propulsion, there are no existing models or empirical data that explain the physics of multi-jet propulsion. As a result, there are no physically motivated rules to guide the design of man-made multi-jet thrusters. In this thesis, I experimentally investigate how interactions between neighboring jets in a multi-jet thruster will affect the system's propulsive performance. I use high-speed fluorescence imaging to investigate the mutual influence of two pulsed jets under conditions relevant to low-speed maneuvering in a vehicle (Re ~ 350). Using a new force estimation technique developed in this thesis, I analyze the video data to evaluate how thrust and efficiency are affected by the jet spacing. This analysis reveals that, compared to non-interacting jets, the efficiency and thrust generated by the pair of interacting jets can fall by nearly 10% as the jets are brought into close proximity. Based on this data, I develop a model of vortex interactions to explain the thrust and efficiency drop. The data and model described in this thesis contribute new insights to understand vortex formation in pulsed jets, and these results can be used to guide the design of multi-jet underwater propulsion systeby Athanasios G. Athanassiadis.S.M

Broadband leaky Lamb waves excited by optical breakdown in water

Optical breakdown of water is used as a sound source to excite a broadband set of leaky Lamb waves in submerged aluminum plates. The source is shown to simultaneously excite guided modes spanning 0.1–5 MHz in frequency and 0–0.8 mm−1 in wavenumber. The measured response overlaps well with dispersion curves for Lamb waves in the plates, revealing strong coupling to both symmetric and antisymmetric modes. The strongest responses arise when a mode's phase velocity approximately equals the plate's compressional wave velocity. These results are shown to arise from an interplay of the sensing geometry, guided wave speeds, and signal processing. Finally, implications for non-contact sensing are discussed.United States. Office of Naval Research (Award N00014-18-1-2066

Effects of multijet coupling on propulsive performance in underwater pulsed jets

Despite the importance of pulsed jets for underwater propulsion, the effect of multiple-jet interactions remains poorly understood. We experimentally investigate how interactions between parallel jets in a pulsed-jet thruster affect the thruster's propulsive performance. Using high-speed fluorescence imaging, we investigate the mutual influence of two pulsed jets under conditions relevant to low-speed maneuvering in a vehicle (Re ≈ 350, L/D ≤ 2). Thrust production and propulsive efficiency are evaluated for different nozzle spacings using a new force estimation technique based on the fluorescence data. This analysis reveals that compared to noninteracting jets, the efficiency and thrust generated by the pair of interacting jets can fall by as much as 10% when the jets are brought into close proximity. Empirically, the thrust T falls off with the nondimensional jet spacing [~ over Δ] as T = T[bar over ∞](1 − Co[~ over Δ][superscript −6]) for a thrust coupling coefficient Co = 2.04 ± 0.11. Finally, we predict this dependence of thrust on spacing using a model that relates the thrust and efficiency drop to streamline curvature and vortex induction at the nozzles.Lincoln LaboratoryUnited States. Office of Naval Research (Award 7000308296