What are Soft Solids and what’s the big challenge?
Soft Solids are materials which appear in a large number of important applications such as in packaging materials, foams and adhesives, paints, foods, rubbers such as tires etc. They are also relevant in Medicine and Biology. Understanding their mechanical behaviour is critical for
- smarter material utilisation,
- efficient and effective material design and
- for achieving desired behaviour properties.
What makes soft solids challenging to work with is that they exhibit inherent complex material behaviour (e.g. non linear viscoelasticity), have complex geometries in their microstructure (e.g. foams), and often include more than one phase (e.g. particulate composite materials). Separating the specific effects of these parameters is extremely difficult and often impossible or not practical experimentally. For example cellular foams used in energy absorbing structures or consumer products such as foods, are affected by the material that constitutes the cell wall (a material effect) as well as the microstructure of the cellular arrangement (a geometry effect). Changing the material of the solid walls without changing the cellular microstructure is neither practical nor cost effective as often changing ingredients leads to different microstructures in the end product. This makes decoupling the material and geometrical contributions to the properties of the end product difficult to achieve without having predictive models that allow parametric analyses. The latter can aid in the understanding of the structure-property relationships present in complex materials and therefore provide a cost efficient and powerful tool for use in product design and optimisation.
My Group’s Work
My group’s expertise lies in combining experimental and modelling techniques to provide complete understanding of how and why a material or structure behaves the way it does. Therefore the Soft Solids group develops predictive models (computational and analytical) that can be used to understand, optimise and enhance soft solid material applications or address performance problems. In parallel, we develop novel experimental methods that can be used to determine important material parameters required as input for the models. In the experimental work we use state of the art microstructural imaging techniques such as SEM including in-situ tests, cryo-SEM, optical microscopy, X Ray Tomography, mechanical characterisation including the high rate regime (speeds from quasi-static to 200 m/s) and full field strain measurements through Digital Image Correlation. We develop models that can:
- replicate mechanical measurements including progressive damage and structure breakdown in soft solids,
- predict the effect of microstructure on the bulk properties; this includes multi-scale models to bridge the various scales often present in soft solids and image based microstructural models,
- analytically predict deformation and fracture in sandwich foams,
- predict interface cracking; these are based on cohesive zone models and non-linear viscoelastic theory.
Examples of our Impact
Recent examples of the impact of our work underscore the importance and diversity of areas where our modelling work brings value:
- the development of multi-scale models for predicting fracture in high filled particulate composites including plastic bonded explosives. Our models are able to predict the correct fracture paths and failure mechanisms allowing virtual testing of materials that are intrinsically difficult to work with and which carry high-risk for experimental testing.
- being able to develop models to predict the adhesive behaviour of drug-loaded patches for treating infection and nail disease. Peeling of the patch is studied at different speeds (to relate this with pain) and with patches of increasing adhesive thicknesses (to relate this with drug-loading capacity). Our work found a novel method for determining the parameters needed in cohesive zone models that are often used to simulate the mechanical behaviour of interfaces such as the one occurring between the nail plate and the patch adhesive. The method is generic such that it can be applied to other interfaces such as the interface between particles and matrix in polymeric composites, the interface between coatings and substrates etc.
- helping art conservationists and museums preserve precious works of art. Our work on assessing risks concerning fluctuations in environment (humidity and temperature) in relation to the integrity of mixed media paintings has highlighted lack of adhesion between different layers of paint as a key variable. Our models are now able to predict crack initiation life times (in years) as a function of fluctuations in the storage environment. As institutions revaluate their environmental control policies in light of the need to conserve energy, prediction of the degradation of works of art becomes more pertinent.
Current and future work
In the last few years, our modelling efforts have had numerous applications in the food industry, including industrial processing of soft solid foods such as dough as well as in studying oral processing. In this space, we have worked on prediction models for structure breakdown of cellular foams and chewing of starch based foods that can provide powerful means for virtual testing of products, avoiding the costly and inefficient trial-and error approach. We are currently working towards the development of predictive models for digestion in order to enable targeted product solutions for nutrition and metabolic health. In addition, we are addressing the oral process more accurately through combining solid-liquid interactions occurring in the oral cavity as well as taking into account the enzymatic breakdown in structures. Our work is essential in determining the food mechanics link to the sensory properties and consumer perception of the foods, as well as to human nutrition.
The Soft Solids group is unique world-wide in that we combine our experimental and modelling expertise in mechanics principles towards understanding how complex soft solids behave under external loads. The importance and the relevance of the work is evidenced through the significant funding we have attracted so far through the UK research councils (EPSCRC, BBSRC), MOD and industry. Since 2010, we have attracted funding in excess of 2.3M.