From interparticle forces to macroscopic yielding of soft amorphous solids

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.

Soft amorphous materials like gels, pastes and dense suspensions are absolutely the stuff of everyday life.

A wide range of foods fall into this category, along with many cosmetics and toiletries. The formulation and development of new products in these areas is built on our existing scientific understanding of soft materials. The gels used in cosmetics or pesticides have been engineered to optimise their shelf life. Similarly, existing understanding of soft matter has been vital when formulating new products like low-fat (or zero-fat) yoghurts, based on accurate size control of emulsion droplets.

This proposal will generate new understanding of the properties of these materials, particularly gels and dense suspensions. There is potential for significant economic and societal impact.


ECONOMY

Soft materials are important in several important economic sectors. Products that rely on such materials include many foodstuffs [UK spend of GBP 134bn/yr (RetailEconomics)], as well as cosmetics [UK market GBP 8.4bn/yr (Cosmetic, Toiletry & Perfumery Association)], hair products [UK market GBP 1.5 bn/yr (EuroMonitor)], and crop protection [Global market GBP 58.6 bn/yr (Markets and Markets)].

For many products within these sectors, a significant challenge is to predict and enhance shelf life. For example, in crop protection or foods (e.g. yoghurts), there is often a microscopic network of particles (a gel) which lends the material some solidity. However, gravitational stresses on this network lead to degradation and, eventually, yielding and failure. The long time-scales involved make this process difficult and expensive to characterise in the industrial context, and we lack theoretical tools required to make predictions about this non-equilibrium process.

This proposal will generate new understanding how gels and dense suspensions fail, including degradation and failure under gravity - this will contribute to the fundamental knowledge required to improve these products. Our study will build on previous work with Bayer AG (see letter of support), related to stability of gels used in pesticides.


POSITIVE ENVIRONMENTAL IMPACT

As well as generating cost savings, improved formulations of soft solids also have environmental impact. For example, the total weight of pesticides applied in the UK alone in 2016 was 16,900 tons (DEFRA), and. If the mechanical properties of these products degrade (for example due to failure under gravitational stress), useable chemical components are wasted, and may be discarded into the environment. Hence, improved formulations that aid stability can reduce waste; control over the rheological properties of the gels can also make their application more efficient, reducing the total amount used. Our research aims to generate new understanding of such processes, with the potential for aiding industrial design.


PEOPLE

The postdoctoral researchers employed on this grant will gain a deep understanding of soft amorphous materials. The technical knowledge of the development of super-resolution imaging techniques and computational simulation and analysis methods, and theoretical understanding gained through the project will provide a broad platform of skills upon which they can base their further careers. These skills will enable the postdoctoral researchers to continue to contribute to the UK knowledge base either through further academic research, or through research in an industrial context.

The investigators are also committed to scientific outreach (see Pathways to Impact) particularly to schools in deprived areas. The everyday nature of the materials studied make the research developments suitable or developing material to inspire the next generation