Date of Award

2022

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Hassan Masoud

Committee Member 1

Jason Blough

Committee Member 2

Hoda Hatoum

Committee Member 3

Jung Yun Bae

Abstract

We designed and experimentally studied the dynamics of two robotic systems that surf along the water-air interface. The robots were self-propelled by means of creating and maintaining a surface tension gradient resulting from an asymmetric release of isopropyl alcohol (IPA). The imbalance in the distribution of surface tension surrounding the robots generates a propulsive force commonly referred to as Marangoni propulsion. First, we considered a single surfer, which was custom-made with novel control mechanisms that allow for both forward motion and steering to be remotely adjusted solely through the manipulation of local surface stresses. We analyzed the performance of this surfing robot characterized by its speed, fuel efficiency, and maneuverability as functions of its operating parameters, such as the release rate and location of IPA. Our robot measures about 110mm in length and can travel as fast as 0.8 body length per second. We found that the linear speed of the robot varies approximately with the discharge rate of the propellant to the power of 1/3. This observation is consistent with a scaling argument derived from the balance of force and energy. Additional maneuverability tests also revealed that the robot is able to withstand 20mm/s2 in centripetal acceleration while turning.

Next, we examined the dynamics of a fleet of robotic surfers traveling together in a coordinated fashion. For this study, we developed a novel rotational array of geometrically simplified surfers that mimics an infinite linear arrangement. The surfers in this idealized system take the form of thin circular disks and are, again, self-propelled via the controlled release of IPA. Utilizing video motion tracking, we analyzed the rotational speed of the fleet as a function of both the spacing between the surfers and the concentration of released IPA. Our analysis was also informed by time-resolved three-dimensional particle tracking velocimetry (PTV) measurements that enabled the visualization of the flow field around the surfers and the hydrodynamics among them. Notably, we found that higher concentrations of the propellant and closer inter-surfer spacing may slow down the surfers. PTV flow visualizations indicated that the surfers’ manipulation of surface stresses for propulsion creates wake vortices and a backward flow stream that increases in intensity with higher propellant concentrations. Destructive interactions with these flow patterns are the likely cause of the observed reduction in the collective propulsion speed of the surfers. Overall, our work provides new insights into the design of Marangoni-driven surfing robots with distinguished functionality and performance and improves the current understanding of the interactive dynamics among a fleet of Marangoni surfers.

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