IRG1:  Dynamics in Soft Interfaces

Faculty Coordinators: J.J. dePablo & H.M. Jaeger

Soft matter comprises a large and diverse class of materials where both the interactions between the constituent molecules or particles and the interfacial energies at their surfaces are small. As a result, such materials can be highly deformable. But beyond that, a special property of many soft matter systems is the explicit feedback between stresses at the surface and the response of the material as a whole. Thus, application of a small stress at a surface not only deforms the material as a whole but can also change its properties, often introducing qualitatively new features. Examples of such behavior abound: a granular material can transform from fluid to rigid when a small change in pressure is applied at a surface; a thin sheet buckles and crumples when a bounding stress is applied; the character of vibration modes inside a soft solid is sensitive to a change in pressure at a surface; strong shear thickening in dense suspensions depends on confinement at the boundaries.

IRG 1 focuses on both scientific challenges and technological opportunities that arise from controlling and manipulating how much or how fast a soft interface forms or deforms. We use the term "interface" to include both exterior, bounding surfaces as well as interfaces that separate regions of different physical properties. We focus particularly on the link between the interface dynamics and the properties of the material as a whole. Thus we will exploit how stress variations at an interface can alter properties in the bulk and, conversely, how tailoring bulk parameters can guide the interface dynamics. This link establishes a theme whose potential for designing specific material responses has come into focus only recently. While this theme applies also to other classes of materials, it becomes especially powerful within the context of soft matter, given the rich set of possiblilities to tune the internal make-up of a soft material and thus optimize its sensitivity of changes in interfaces.

In Focus Area 1, we explore how interface manipulation can drive large changes in the ratio of bulk modulus B to shear modulus G of a soft material in order to create compliance that can vary and adapt to different loading conditions. This effort investigates the interplay of interface dynamics and bulk behavior for both slow deformations and their more rapid, rate-dependent counterparts. Even in very deformable materials there are, however, limits to mechanical compliance and eventually a system will become unstable or break apart. These issues are investigated in Focus Area 2, where our goal is to control the behavior during instabilities or at the point of catastrophic deformation, exploiting the associated failure modes for shaping specific stress responses.


By exploiting the dynamics of soft-material interfaces to create and control specific behavior, this IRG breaks new ground. Considering interfaces across a wide range of materials, from nanoscale colloids to macroscopic granular materials, the IRG lays the foundation for a comprehensive understanding of the underlying fundamental science while providing a path toward new applications.

Guiding crack propagation. Top: Experimental setup and sketch of positive and negative Gaussian curvature. Bottom: 4 results showing how regions of positive Gaussian curvature are avoided by cracks.
Optical microscopy images of wrinkling and buckling in films of (a) dodecanethiol-capped Au nanoparticles and (b) functionalized cross-linked nanoparticles, as they are compressed and then expanded.
Amorphous topological insulators constructed from random point sets. Left: Individual chiral element. Right: Amorphous network from the magnetic coupling of individual elements to nearest neighbors.
Network-based, amorphous metamaterials. Top: 2-d experiment and 3-d simulation. Bottom: Optimized pruning strategy. Color indicates ordering of minimal cycle areas.
Drop impact on a flat substrate and splashing onset, including emergence of a thin liquid sheet. (a)-(c) show high-speed side view images; (d)-(f) are simultaneously recorded bottom-view images.
Snapshots from time evolution of internal flow field underneath impacted region, showing propagating shear-jamming front (edge of green region).