Session R30: Visualizing Forces in Soft Materials via Photoelastic and Other Optical Techniques

Stress and velocity fluctuations in photoelastic granular avalanches

We study granular avalanches using a custom-built narrow chute where we release 2D photoelastic disks down an incline. Using high-speed imagery, we are able to obtain position and velocity data from particle tracking, and the full stress tensor, including normal and shear stress components, from the photoelastic response of interacting particles. Even though the avalanche is steady-state in time and space, minute fluctuations in velocity and forces away from the mean directly influence the rheology and fluidity.

In this study, we analyze the correlation between velocity fluctuations and stress fluctuations in both the quasi-steady layer (close to the rough base) and the flowing layer (near the free surface). We correlate the fluctuations with direct measurements of the measured non-local properties within this photoelastic avalanche.   

Photoelastic Analysis of Cohesive Granular Aggregates

Photoelasticimetry facilitates the non-invasive measurement of individual contact forces in granular materials. This gives us greater insight into the spatially hetergeneous structure of force transmission characteristic of amorphous granular solids. To our knowledge, photoelasticimetry has been used exclusively to measure contact force networks in aggregates of repulsive particles. We present photoelasticimetric force measurements of systems of particles which exhibit attractive interparticle forces, and contrast the structure of force networks in these systems with the networks in purely repulsive aggregates.   

Optical characterization of underwater contact mechanics

For survival in extreme environments, organisms have evolved adhesive mechanisms and materials. While chemistry of underwater bio-adhesion is a source of valuable insight, the mechanics by which surfaces expel water and come in contact underlies critical understanding of natural solutions and evaluating biomimetic analogs. Hydrophobicity of an adhesive surface has been shown to be crucial in removing water from a hydrophilic substrate, but the resulting contact is typically heterogeneous, with patches of unevacuated water. Here, we present a simple, FTIR-based imaging technique to spatially resolve and quantify thickness of nanoscopic puddles formed between two solids in contact under water. The technique is validated by comparing measured air gap thickness of a glass lens in contact with glass prism with Hertzian contact theory, and then applied to characterize the drainage and formation of patches of water as a soft PDMS lens approaches a glass surface at varying speeds. The work paves the way for better characterization of interfaces in contact under water and can find application in adhesive development, biological study and tribology.

reference: M. Shirota, M. A. van Limbeek, D. Lohse and C. Sun, Eur. Phys. J. E: Soft Matter Biol. Phys., 2017, 40, 54   

Worms in Jell-O: Using photoelastic stress analysis to measure burrowing forces

Muddy marine sediments are elastic materials through which worms and other animals extend burrows by fracture. Gelatin has similar fracture properties to muds, and worms burrow readily through this transparent analog. Polarized light shows that the water-filled burrow has an elongated, tongue-depressor-shaped crack. Photoelastic stress analysis was used to measure forces applied by worms burrowing in gelatin. These results, combined with modeling, show that stress intensity factors at the crack tip reach the fracture toughness, validating the method of burrow extension by fracture. Additionally, these measured forces and understanding of the mechanics of burrowing allowed for quantification of the energetic cost of burrowing, which is much lower than previously thought. Stress visualization in gelatin has also been used to test instruments to measure fracture toughness and stiffness of marine sediments. These properties govern burrowing behaviors and may be important metrics to quantify the ecosystem engineering impacts of animals living in sediments.   

Exploiting photoelasticity to characterize dynamics of polymer networks

The development of internal stresses within polymer networks is a major factor to consider when utilizing this class of soft materials, particularly when compared to their linear analogues. As an example, internal stresses that develop during network polymerizations often lead to macroscopic failures such as delamination or cracking when networks are constrained at one or multiple interfaces. To better understand and mitigate stress behavior, macroscopic mechanical characterization techniques (e.g. tensile testing, rheometry) have typically been employed. However, these methods provide little to no insight as to the dynamics of stress evolution on a local level, nor how internal stresses evolve in response to environmental changes. In this work, we exploit the photoelastic behavior of polymer networks to characterize internal stresses that develop during swelling and subsequent rupture of hydrogels. Compared to the majority of studies into mechanics of hydrogels, which typically characterize materials in one of two separate states (dry or swollen), this approach enables a non-invasive, real-time assessment of internal stress development during swelling. We discuss how this approach can better inform soft materials developed for dynamic environments.
  

The local mechanics of macroscopic heterogeneous photoelastic polymer networks

Stress localization and fracture within networks are common observations but predicting failure relies on a complex interplay between each connected component or beam. Simulations on model networks have shown intriguing connections between connectivity and stress. But do these results translate into physical experiments? To explore this, we produce macroscopic (~cm), heterogeneous (removing beams or varying beam thickness), 2D networks by using a 3D printed mold that is cast first with a silicone rubber, and then with a highly responsive photoelastic resin. Upon straining these networks to failure, each beam experiences a local stress resulting in a proportional photoelastic signal which is captured with a camera while measuring the bulk force with a capacitive load cell. This signal is compared to a previously measured force-intensity calibration curve from a single beam. Concomitantly, a second camera captures brightfield images, used to calculate the local strain. This local stress/strain information in programmable heterogenous networks is used as input for spring network simulations. We will present the comparisons between simulations/experiments and explore the underlying mechanics of a polymer network in both the linear and non-linear regime just before network failure.   

Direct force measurement of microscopic droplets pulled along soft surfaces

Recently, there has been growing interest in understanding the interactions of liquid droplets with soft materials. In these systems, forces exerted by a droplet can deform the material it contacts. This property leads to a plethora of unique physical phenomena with applications in fields ranging from water collection to surface sensing. We explore droplet dynamics on soft materials using a micropipette-based technique to simultaneously image, and measure the forces on, a microscopic droplet dragged along a soft interface. By changing the thickness of the soft material, we can control the compliance independent of surface chemistry. We show that material stiffness is a key parameter in droplet dynamics.   

Experimental Studies on Slow Impacts and Interaction with Regolith Covered Surfaces in Low Gravity

Splashes from slow impacts into granular materials play an important role in the sculpting of asteroid surfaces and for the structures formed in wind blown sands. This talk I will give an overview of some recent experiments that explore ejecta generation and material redistribution from slow impacts into a granular bed under realistic space condtions. Most prominently an experiment which shows that ejecta created by slow impacts will stay close to the impact site under conditions realistic for a small asteroid, which has implications for observed size sorting effects. However this experiment also found that ejecta will not always be created directly at the impact site. By using photoelastic materials it was possible to identify that subsurface buckling introduced by the impact is responsible for this effect. Also, using similar techniques, it is possible investiage the ejecta creation process when mechanically interacting with planetary surfaces, e.g. landing or sampling, with the goal to identify an interaction design that will minimize ejecta creation.
  

Optical spatio-temporal control of active matter systems in 2-D.

Active matter systems consist of self-driven units which convert chemical energy into mechanical energy and organize themselves into various patterns. Their pattern and flow properties exhibit non-equilibrium properties. In our experiments, we investigate the pattern formation of microtubule, kinesin motor kinesin motor proteins in 2-D fluid/fluid interfaces. The kinesin motor proteins are engineered in a way that light induces reversible linkage between them. Thus, under the exposure to the light (blue light centered at wavelength 460nm) the end groups of kinesin motor proteins bind and create the sliding motion between the microtubule bundles. Due to topological constraints, bundles create + ½ (motile) and - ½ (non-motile) defects. Using various light patterns, we confine the sample both in space and time. Illuminated regions exhibit strong bundling where the flow patterns and speed of the + ½ defects depend on the power and the design of the light pattern. Our goal is to control the defect velocity and their direction to guide them towards a certain destination.
  

Dynamical Inference of Forces Using Three-dimensional Tomographic Imaging of Dusty Plasmas

Charged, micron-sized particles can be suspended in a in a low-pressure plasma through a balance of gravitational and electrostatic forces. Known as a dusty plasma, the particles are also subject to a screened, Coulomb repulsion, in addition to a number of kinetic and hydrodynamic drag forces from the ambient environment, leading to a rich array of nonequilibrium collective behavior. Using a new, high-speed, three-dimensional imaging technique based on laser-sheet tomography, we are able to track the dynamics of up to 50 individual particles over tens of seconds. In small systems with 2-3 particles, normal-mode decomposition provides a quantitative estimate of the equilibrium charge of the particles, the Debye length in the plasma, and the local electric field. For larger systems with many particles, the data also allows us compute the average pairwise forces between particles as a function of separation. The method is robust to noise in the data, and can be applied to other many-body systems with complex interactions.