Soft Materials

This research focuses on the properties of common soft materials. We study colloids, emulsions, drops, polymers and gels. Our focus is on understanding the structure of these materials and how this controls their dynamics and properties. We make extensive use of the experimental tools of a soft matter physics lab, including optical microscopy, light scattering and rheology. We also invent new tools and experimental techniques to explore all properties of the material. Much of our focus is on the mechanical properties of the materials and their relationship to the internal dynamics of the structures within the material. Our work is often motivated by technological applications of the materials, and we work with industrial partners to help them solve important problems. In addition, we search for new ways to create materials with interesting properties and high value. We also use soft materials, such as colloidal particles or microgels, as model systems to investigate the behavior of more complex matter.

Fluid transfers and deformations in sub-micron porous media: Porous materials usually contain moisture in equilibrium with their surroundings. This liquid may be subjected to changes in atmospheric conditions such as a variation in temperature, humidity, external pressure or air velocity etc. which inevitably result in the material losing or gaining moisture to re-establish equilibrium with its environment. These transitioning mechanisms are named after drying for fluid removal and imbibition and adsorption for fluid sorption.  Familiar to anyone who ever filled a sponge with water and left it to dry, these transitions can affect the aspect, the integrity and the durability of the material. Using model and homogeneous porous systems with pore size ranging from a few microns to a couple of nanometers, we seek to identify the physical origin of these deformations as a function of the local saturation state of the material as well as the flow properties of the liquid. Our work benefits from high resolution MRI and Electron microscopy measurements and is co-advised by Philippe Coussot ( Jules Thiery

Mechanics of microcapsules: Microcapsules can be easily processed by microfluidic techniques. One important application is to encapsulate cargos for delivery and controlled release. The mechanical stability of microcapsules is of special concern. The microcapsules are expected to be stable during processing, while not permanently stable during release. My research is to understand the mechanical behaviors of microcapsules, and in further to harness “when” and “how” to trigger the breakup. We have successfully created various microcapsule structures and correlated these structures to the stability control. Our fundamental understanding is to serve the application of antibody delivery, ultimately, for disease treatment in human body. Weichao Shi 

Mechanical properties of filled rubber: Filled rubbers are composite materials containing two interpenetrating phases: crosslinked elastomers, and a ‘filler’ consisting of colloidal particle aggregates.  Above a critical volume fraction, the colloidal aggregates form a system-spanning subnetwork that reinforces the elastomer network and introduces a new energy loss mechanism at low strains of only 1-5%. This low-strain energy loss mechanism, known as the Payne Effect, is one of the mechanical hallmarks of filled rubbers and is a major contributor to rolling friction in tires. The physical origin of the Payne Effect is linked to the structure of the filler subnetwork and the ultimate breakdown of the subnetwork under applied shear strain. However, current techniques fail to describe the specifics of how the filler network responds dynamically while undergoing shear strain.  Zach Gault

Gelation and phase separation: We study the aggregation, gelation and phase separation of micron-scale colloidal particles made attractive with the addition of non-adsorbing polymer depletant. We observe morphology changes as a result of the range and strength of the depletion attraction, and that the kinetic arrest in gelation is driven by the process of phase separation. Peter Lu
Drying of complex suspensions: Mixtures of immiscible fluids with colloids can be very complex but they are technologically important for industries such as paints and protective coatings, especially when such materials undergo drying. Emulsions containing colloidal particles are particularly interesting as controllable test cases of such systems, but they are difficult to image because these mixtures typically scatter light strongly. We use confocal microscopy to understand full 3D picture of what happens when these emulsions dry out. Peter Lu, with Lei Xu
Phase Separation in Microgravity: When a liquid separates from a gas of the same material on earth, the denser liquid invariably sinks. In the microgravity environment of the International Space Station, however, this effect is reduced by six orders of magnitude. As a result, we can observe the spatial patterns that form in near-critical liquid-gas phase-separating mixtures for weeks, orders of magnitude longer than what can be done on earth. We work in collaboration with a number of astronauts onboard the ISS. Peter Lu

Rheological behavior of graphene oxide aqueous suspensions: Graphene materials are now receiving great attentions from both academic and industrial fields because of their promising applications; for example, they are used as three-dimensional printing ink, applied as coating and contained in polymer nanocomposites and energy storage materials. The fabrication of graphene materials through a wide range of industrial techniques requires control over the flow behavior as well as the viscosity and elasticity of two-dimensional (2D) graphene oxide (GO) suspensions, the most important precursor of graphene. However, the rheological behavior of this 2D material remains less explored compared to 0D spherical particles. To explore the rheological behavior and ordering effects for suspensions of GO nanosheets, we developed a model system using GO nanosheets with controllable lateral/thickness ratio ranging from ~10 to ~1000. Liangliang Qu

Asymmetric Liposomes and Lipid-Polymer Hybrids: We work on fabricating asymmetrical liposomes and polymer-lipid hybrid vesicles, which are aqueous volumes enclosed by a bilayer consisting of dissimilar lipids/polymers in each monolayer, through new kinds of microfluidic devices. Asymmetrical lipid/polymer vesicles are interesting because they can help optimizing drug delivery process and also allow scientists to study the fundamental biophysics related to asymmetry and understand more about why eukaryotic cell membranes are all asymmetric. We specifically use microfluidics to fabricate these asymmetrical vesicles resulting in homogeneous sample in both composition and size. The microfluidics technology to design homogenous samples of asymmetrical vesicles with high-throughput and precision. Yuting (Tina) Huang

Mechanical deformation of colloidal glass. While the bulk material response of a disordered glassy material to external loading is well characterized for a wide range of hard and soft materials, the detailed microscopic mechanism governing the local deformation of disordered materials is poorly understood. I use hard sphere colloidal glass under mechanical perturbations as a model system to study the microscopic mechanisms that collectively give rise to the bulk plastic flow in metallic glasses. I am interested in coupling local free volume and the observed evolution of the strain field within the colloidal glass by combining globally applied piezo-actuated strain deformation and locally applied force on isolated magnetic probes embedded in the glass. Zsolt Terdik

Oscillating chemical reactions. Oscillating reactions are among the most fascinating chemical reactions. In a typical oscillating reaction, the concentration of one or more components exhibits periodic changes. These periodic changes of concentration could lead to the redox potential oscillation, color oscillation or mechanical oscillation of the system. Oscillating chemical reactions are important in life and probably played a central role in origin of life. For example, cell division, circadian rhythms and nerve impulse all involve periodic reactions. We are interested in building an artificial oscillating chemical system to mimic biological system and investigating the fundamental mechanism of life and its origin. Yongcheng Wang

Flow in Binary Colloidal Glasses: Glassy materials are of great importance both as a subject of scientific study and also for their enormous potential in engineering applications. Distinctive feature of glasses is the absence of long-range periodic order in their internal structure along with low atomic or molecular mobility. An essential unresolved question in the physics of glasses is a comprehensive understanding of their behavior under applied stress. How do the constituent molecules or atoms in a glass respond to an external force? Such a study would be highly challenging in a material like window glass. Molecular movement within a sheared window glass is too fast to be tracked easily by conventional microscopes. Therefore, a system with a much slower response time would be desirable. It is known that the time scale of the motion of colloids is slow enough to render their dynamics trackable in real time. Also, their size is large enough to make them visible in an optical microscope. Colloids can be synthesized with high precision and nearly perfect size dispersity with tunable inter-particle interactions. Therefore, in this study, we are aiming at making a glassy system based on two different colloidal particles, reminiscent of glassy alloys, which can be exposed to external shear with simultaneous investigation under optical microscope. This study is expected to provide more insights into the displacement of the glass constituents upon macroscopic deformation. Payam Payamyar