Spring 2018 Schedule

Date SpeakerLab
01/25/2018Erin McCaskey
Christian Hubicki
Goldman Lab
02/01/2018Jeff GauSponberg Lab
02/08/2018Yu-Hui LinWeitz Group
02/15/2018Ben KalziqiYunker Lab
02/22/2018Jessica FaubelCurtis Lab
03/01/2018Ashley CoenenWeitz Group
03/08/2018APS March Meeting
03/15/2018Bo LeeHu Lab
03/22/2018Spring Break
03/29/2018Joy PutneySponberg Lab
04/05/2018Jonathan MichelYunker Lab
04/12/2018Derek HartKim Lab
04/26/2018Karl LundquistGumbart Lab

Spring 2018 Abstracts


Christian Hubicki and Erin McCaskey

Biological and Robophysical Investigation of Root Circumnutation through Heterogeneous Substrates

Circumnutation, the circular motion exhibited by the tip of a growing plant root, is exhibited by a variety of plant species, but its function is not well-understood. We hypothesize that the circumnutation characteristics of these plant species result in greater success in growing around substrate heterogeneties (e.g. obstacles embedded in soil). In a recent discovery by the Benfey Lab at Duke University, mutated rice species’s roots were grown showing different circumnutation characteristics, even greatly reducing circumnutation. This manipulation allowed us to experimentally test the performance of root growth strategies in both a circumnutating species and its non-circumnutating mutant counterpart. We grew both rice variants in a gel substrate laden with rigid obstacles. To monitor multiple simultaneous root growth trials, an automated picture acquisition system was built to obtain videos of different rice species growing in the gel substrate. Preliminary experiments conducted show promising support of our hypothesis, but current work is being done to increase the data set and quantitatively measure circumnutation characteristics. To more-systematically investigate the mechanics of circumnutation in heterogeneous substrates, we constructed a simple robophysical model of a growing root. Preliminary experiments show that circumnutation in the “robo-root” reduces the probability of obstacles physically halting growth progress.


Jeff Gau

Mechanics of insect flight

Flapping flight at the centimeter scale is one of the most energetically demanding modes of locomotion. Flying insects have been thought to have evolved a resonant flight system to improve energy economy. However, the actuators (muscles) in insects have strong strain dependencies. We hypothesize that this coupling between forcing and kinematics gives rise to an unexplored class of dynamical systems. We began by characterizing the dynamic mechanical properties of the thoracic exoskeleton (transmission between muscle and wings) in the hawkmoth Manduca sexta. We isolated the exoskeleton and drove sinusoidal length sweeps from 0.1 to 90 Hz with physiological amplitudes. We find that a linear model captures the bulk mechanical properties of the heterogeneous thorax. The thorax was 75% elastically efficient across a wide range of frequencies and amplitudes. This lead to a total body-mass specific power return of 6 W kg-1, which reduces inertial power demands by 20%. With this mechanical representation of the flight apparatus, we can begin to understand how the coupling between strain-dependent actuators and deformable structures affects flapping flight dynamics.


Yu-Hui Lin

The effects of spatial interactions on an oscillatory tragedy of the commons

Game theory has long been an essential tool for the study of evolutionary dynamics of biological systems. However, the study of evolutionary game theory rarely considers the joint dynamics of how individual strategies shape the environment as well as how payoff of strategies differ in response to changing environments. To address this joint dynamics, Weitz et. al. recently proposed a nonlinear model of coevolution between individual strategies and environment [1]. The model exhibits a range of dynamical behaviors in the joint strategy-environment state space. Of particular interest is the emergence of an “oscillatory tragedy of the commons”, in which the system oscillates between extremal states due to the feedback of individual strategies.

Nonetheless, the mean-field ODE treatment does not account for the effect of population noise and local interactions, which are present in real ecological systems. To include such effects, we derived individual-based game rules based on the ODE model [1] and implemented them in spatially explicit simulations, where individuals can only interact with others and the environment locally. In doing so, we found that including explicit spatial interactions in the model averts a tragedy of commons in a wider range of parameter regimes compared to the ODE model [1]. In addition to stabilizing the system, local interactions also lead to novel spatiotemporal dynamics. The individual-based, spatially explicit model displays coherent patterns, including dynamic clusters and wave-like invasions, that emerge from an initially well-mixed population. Studying this model may provide insights into possible mechanisms leading to the tragedy of the commons and conditions for which the tragedy can be averted.

[1] Weitz, Joshua S., et. al. “An oscillating tragedy of the commons in replicator dynamics with game-environment feedback.” Proceedings of the National Academy of Sciences 113.47 (2016): E7518-E7525.


Ben Kalziqi

Killing to Fluctuate

Unlike equilibrium atomic solids, biofilms—soft solids composed of bacterial cells—do not experience significant thermal fluctuations at the constituent level. However, living cells stochastically reproduce and die, provoking a mechanical response. We investigate the mechanical consequences of cellular death and reproduction by measuring surface-height fluctuations of biofilms containing two mutually antagonistic strains of Vibrio cholerae that kill one another on contact via the type VI secretion system. While studies of active matter typically focus on activity via constituent mobility, here, activity is mediated by reproduction and death events in otherwise immobilized cells. Biofilm surface topography is measured in the nearly homeostatic limit via white light interferometry. Although biofilms are far from equilibrium systems, measured surface-height fluctuation spectra resemble the spectra of thermal permeable membranes but with an activity-mediated effective temperature, as predicted by Risler, Peilloux, and Prost [Phys. Rev. Lett. 115, 258104 (2015)]. By comparing the activity of killer strains of V. cholerae with that of genetically modified strains that cannot kill each other and validating with individual-based simulations, we demonstrate that extracted effective temperatures increase with the amount of death and reproduction and that death and reproduction can fluidize biofilms. Together, these observations demonstrate the unique physical consequences of activity mediated by death and reproduction events.


Jessica Faubel

Biomimetic Hyaluronan Polymer Brush Grown from Enzymes