Soft Matter

A mass of entangled granular media, interconnected staples.

In our lab, we study the rheology of complex granular materials to understand the fundamental physics of limb-ground interactions during locomotion. While considerable work has been done to understand the physics of animal and robot locomotion on hard ground, softer substrates like granular media are commonly encountered in nature.

 

Uncovering the physics of how limbs interact with sand is fundamental to understanding animal locomotion and making informed control decisions in robotic actuation.

 

Below is a sampling of our contributions, including avalanche dynamics, granular impacts, packing, nonsherical granular entanglement, and granular drag and intrusion forces.

Acceleration vs time of a sphere during impact with granular media, Scaling and Dynamics of Sphere and Disk Impact into Granular Media, Daniel I. Goldman and Paul B. Umbanhowar, PRE, 77, 021308 (2008).

Impacts in Granular Media

Impact of objects into substrates like loose sand is relevant in many settings including ground-foot interaction during rapid surface locomotion. Unlike well-formed granular flows which may be described by hydrodynamic-like equations, a comprehensive description impulsive dynamics is not available. We studied the pronounced and nontrivial dependence of impact dynamics on granular volume fraction and intruder geometry.

 

Granular impact and the critical packing state, Paul B. Umbanhowar and Daniel I. Goldman, PRE Rapid Communications, 82, 010301R, (2010)

Scaling and Dynamics of Sphere and Disk Impact into Granular Media, Daniel I. Goldman and Paul B. Umbanhowar, PRE, 77, 021308 (2008)

A mass of entangled granular media, interconnected staples.

Entangled Granular Media

Many living (ants and cells for example) and nonliving (rods, nonconvex particles, etc) materials are composed of ensembles of nonspherical particles. The shape of grains in granular materials can significantly affect their collective physics. In this paper, we studied the stability of piles formed from concave, “u”-shaped particles.

 

Entangled Granular Media, Nick Gravish, Scott V. Franklin, David L. Hu, and Daniel I. Goldman, Physical Review Letters, 108, 208001 (2012)

 

Video

Experimental setup for avalanching experiments, The effect of volume fraction on granular avalanche dynamics, Nick Gravish, and Daniel I. Goldman, Physical Review E, 90, 032202 (2014).

Avalanching Dynamics

A granular avalanche is an important example of the granular solid-to-fluid transition. Avalanches of granula media are ubiqutious, occuring in industrial, laboratory, and natural settings (landslides). The evolution and failure of a granular slope as a function of granular medium compaction (the volume fraction) is studied using a rotating bed.

 

The effect of volume fraction on granular avalanche dynamics, Nick Gravish, and Daniel I. Goldman, Physical Review E, 90, 032202 (2014)

Experimental setup for side-wall drag experiment: plate is placed against a transparent sidewall and high-speed video is recorded, Force and flow at the onset of drag in plowed granular media, Nick Gravish, Paul B Umbanhowar, and Daniel I. Goldman, Physical Review E, 89, 042202 (2014).

Drag in Granular Media

Drag and lift forces in aerodynamics are critical for airplanes to take off. In granular media, they affect how animals swim and burrow. In the following studies, we explored the dependence of forces on the intruder's shape and packing state.

 

Force and flow at the onset of drag in plowed granular media, Nick Gravish, Paul B Umbanhowar, and Daniel I. Goldman, Physical Review E, 89, 042202 (2014) 

Drag induced lift in granular media, Yang Ding, Nick Gravish and Daniel I. Goldman, Physical Review Letters, 106, 028001 (2011)

Force and flow transition in plowed granular media, Nick Gravish, Paul B. Umbanhowar and Daniel I. Goldman, Physical Review Letters, 105, 128301 (2010)