From interparticle forces to macroscopic yielding of soft amorphous solids

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The response of a material to mechanical loading is one of its most basic properties. Under sufficient load, materials yield and fail, often in a catastrophic fashion. This macroscopic behaviour is ultimately governed by the particles that make up the material - for example, its constituent atoms and molecules. In many applications, one would like to predict a material's macroscopic behaviour, starting from its microscopic constituents. We propose to study the links between microscopic and macroscopic properties of a class of soft materials. These materials are assembled using microscopic particles much bigger than atoms, and interact more weakly. Thus the cohesive forces that hold the particles together are much weaker: the materials are soft and can be easily deformed by mechanical loads. Moreover, in contrast to many familiar materials, the arrangement of the particles is amorphous, and does not resemble the ordered crystals that one typically finds in metals or minerals.This class of material includes gels, as used in products like pesticides, cosmetics, or food. After being prepared, gels degrade with time and eventually become so unstable that they collapse under their own weight. This limits the shelf-life of many products - by analysing the degradation process and linking it to the microscopic behaviour, we hope to inform the design and formulation of future products.Our proposed research will focus on gels formed from emulsions, which consist of microscopic droplets of oil suspended in a watery medium. Milk is an example of such an emulsion. However, the emulsion that we will use has been tailored to allow new kinds of measurement: our emulsion droplets include special fluorescent dye molecules which respond to a mechanical load. We use an extremely powerful microscope (sometimes called a "nanoscope") to see dye molecules at the contact point between two adjacent oil droplets. The more the droplets are squeezed, the brighter the light from the dye molecules.Depending on the experimental conditions, we can assemble these droplets into a network (a gel) or pack them until they almost touch to form a "glass". We will study these amorphous solids, under mechanical load. Together with the dye, our nanoscope will allow us to measure the forces between these microscopic droplets. This kind of measurement has not been possible until now, and will give us vital new information as we analyse the links between the particles' behaviour and the macroscopic properties of the gel.Our experiments will be compared with computer simulations, which provide accurate microscopic descriptions of these materials, without the difficulties associated with imaging small droplets. But there are restrictions on the system sizes and time scales such microscopic simulations access, due to limited computational resources. We will combine simulation and experiment, which provide complementary information - the simulations are accurate on small scales while the experiments reveal the behaviour of macroscopic systems. The experiments will be tested and calibrated against the simulation data.In this way, we answer two kinds of question. First, we understand what happens as a material yields, either under its own weight (gels) or as it flows in response to an external force (glasses). For example, where are the weak points where these materials fail? Can this process be controlled? Second, we will compare our results with theoretical predictions to understand the principles that govern the properties of these materials. Different theories make different assumptions, and make a range of predictions about how amorphous solids yield and flow, and how this depends on their microscopic structure. Our experimental measurement of forces will provide detailed information about these colloidal systems, allowing us to test the theoretical predictions in new ways, and - we hope - to uncover new physical behaviour.