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ItemGeometric modeling of biological and robotic locomotion in highly damped environments(Georgia Institute of Technology, 2022-12-14) Zhong, BaxiBiological systems can use seemingly simple rhythmic body and limb undulations to traverse their complex natural terrains. We are particularly interested in the regime of locomotion in highly damped environments, which we refer to as geometric locomotion. In geometric locomotion, the net translation is generated from properly coordinated self-deformation to counter the drag forces, as opposed to inertia-dominated systems where inertial forces dominate over frictional forces (thus coasting/gliding is possible). The scope of geometric locomotion include locomotors with diverse morphologies across scales in various environments. For example, at the macroscopic scale, legged animals such as fire salamanders (S. salamandra), display high maneuverability by properly coordinating their body bending and leg movements. At the microscopic scale, nematode worms, such as C. elegans, can manipulate body undulation patterns to facilitate effective locomotion in diverse environments. These movements often require proper coordination of animal bodies and/or limbs; more importantly, such coordination patterns are environment dependent. In robotic locomotion, however, the state-of-the-art gait design and feedback control algorithms are computationally costly and typically not transferable across platforms and scenarios (body-morphologies and environments), thus limiting the versatility and performance capabilities of engineering systems. While it is challenging to directly replicate the success in biological systems to robotic systems, the study of biological locomotors can establish simple locomotion models and principles to guide robotics control processes. The overarching goal of this thesis is to (1) connect the observations in biological systems to the optimization problems in robotics applications, and (2) use robotics as tools to analyze locomotion behaviors in various biological systems. In the last 30 years, a framework called “geometric mechanics” has been developed as a general scheme to link locomotor performance to the patterns of “self-deformation”. This geometric approach replaces laborious calculation with illustrative diagrams. Historically, this geometric approach was limited to low degree-of-freedom systems while assuming an idealized contact model with the environment. This thesis develops and advances the geometric mechanics framework to overcome both of these limitations; and thereby generates insight into understanding a variety of animal behaviors as well as controlling robots, from short-limb elongate quadrupeds to body-undulatory multi-legged centipedes in highly-damped environments.
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ItemNon-inertial Undulatory Locomotion Across Scales(Georgia Institute of Technology, 2022-12-13) Diaz Cruz, KelimarLocomotion is crucial to behaviors such as predator avoidance, foraging, and mating. In particular, undulatory locomotion is one of the most common forms of locomotion. From microscopic flagellates to swimming fish and slithering snakes, this form of locomotion is a remarkably robust self-propulsion strategy that allows a diversity of organisms to navigate myriad environments. While often thought of as exclusive to limbless organisms, a variety of locomotors possessing few to many appendages rely on waves of undulation for locomotion. In inertial regimes, organisms can leverage the forces generated by their body and the surrounding medium's inertia to enhance their locomotion (e.g., coast or glide). On the other hand, in non-inertial regimes self-propulsion is dominated by damping (viscous or frictional), and thus the ability for organisms to generate motion is dependent on the sequence of internal shape changes. In this thesis, we study a variety of undulating systems that locomote in highly damped regimes. We perform studies on systems ranging from zero to many appendages. Specifically, we focus on four distinct undulatory systems: 1) C. elegans, 2) quadriflagellate algae (bearing four flagella), 3) centipedes on terrestrial environments, and 4) centipedes on fluid environments. For each of these systems, we study how the coordination of their many degrees of freedom leads to specific locomotive behaviors. Further, we propose hypotheses for the observed behaviors in the context of each of these system's ecology.
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ItemField-mediated interaction in active matter(Georgia Institute of Technology, 2022-04-28) Li, ShengkaiLike most physical systems, the interaction characteristics among agents play an important role in active matter. For example, the extent of attraction can switch a collective of particles from a homogeneous mixture to phase-separated clusters; particle concavity in shape-changing active systems can change interactions from repulsive to attractive. The way that the force transmits can also be important. While many interactions transmit through direct or short-ranged contact (e.g., collisions or magnetic attraction), there are interactions that require the full description of the force-generating field to describe motion. These interactions can bring interesting features such as time delays, the coexistence of multiple length scales, and non-reciprocity, which are less common in short-ranged interacting systems. In this thesis, I will use several examples from my Ph.D. work to show the rich dynamics of active matter interacting through a field. Examples include active locomotors mimicking motion in curved space-time when driving on an elastic membrane, and resource-consuming agents driven by resource depletion that form different states of matter. Through these studies, I will also show how the connection between field-mediated interactions and classical fields allows us to explain and explore emergent phenomena in active matter using inspiration and tools from field theory. In addition to the study of field-mediated interactions, other studies of active matter with short-ranged interactions are presented in the later chapters. These include shape-changing active matter, the role of substrate curvature in active matter, and the analogy between attractive interaction in active matter and surface tension.
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ItemRate-dependent Locomotion and Intrusion Phenomena in Granular Media(Georgia Institute of Technology, 2022-04-28) Karsai, AndrasIn movement on solid terrains, animals, vehicles, and robots can make use of well-established contact dynamics for planning movement and locomotion gaits. However, when the terrain can be deformed significantly, effective traversal can be inhibited by terrain heterogeneities created before and/or during locomotion. Understanding the physical behavior of such deformations such complex forcings like locomotion can inform robotic navigation strategies and expand our physical intuition of soft matter substrates. We examine various rate-dependent phenomena for a specific class of flowable substrates abundant in the natural world: granular media, which exhibit multiphase and hysteretic properties as a collective of many small rigid bodies. The physics of granular substrates is dominated by a network of frictional contacts between simple particles, which nevertheless display a wealth of unexpected multiphase phenomena depending on their stresses and packings. We present a series of experimental studies on such media. An anthropogenic mode of terrain traversal, wheeled locomotion, can locomote via the reaction force generated from actively shearing a granular substrate. We experimentally show how such locomotion can induce rate-dependent weakening via the centripetal acceleration of the media under shear and present a fundamental physics-based cause for why vehicular slippage occurs at high wheel rotation rates. In another robophysical study, we demonstrate how a small rover robot can effectively remodel steep granular slopes via strategic open-loop gait selection. By selectively avalanching frictional media towards itself, the robot could traverse loosely consolidated granular hills that otherwise would not be possible for it to climb. Finally, we investigate how directional fluidization of granular media during intrusion could modulate the resistive forces to allow a body to move efficiently within a substrate that constantly attempts frictionally hinder it. This dissertation showcases not only the strength of experimental investigation in physics to discover new phenomena, but also how simple reduced-order models can be adapted to explain them and synthesize our observations for applications in robotics, terradynamics, and more.