Each year, SCI’s Scotland group runs a competition where students are invited to write a short article describing how their PhD research relates to SCI’s strapline: where science meets business.
David Crosby, a PhD student at the School of Physics and Astronomy from the University of Edinburgh, was the runner up in this year’s competition. His article ‘The Physics of Formulations’ is reproduced here.
The Physics of FormulationsOne of the most common, and often dreaded, questions one can ask a physics PhD student is ‘Can you explain the real-world significance of your research?’ This is perhaps difficult to answer for more abstract research such as looking into the evolution of galaxies or uncovering the mysteries of particle physics. However, when I am asked this question I respond with ‘My research involves investigating novel ways of formulating skin creams.’
In broader terms my research falls into the category of soft matter physics. Officially soft matter physics deals with materials which can be altered by thermal fluctuations, and lies at the interface of chemistry, physics and biology. Everyday life is filled with examples of soft matter such as condiments in the kitchen or personal care products in your bathroom. The design of multifunctional soft matter is one of the aims of both industrial and academic research. My own research being an example, I study a novel soft matter system called surface active microgels, which can both thicken and stabilise components of complex fluids.
Microgels are soft colloidal particles composed of a cross-linked network of polymers. An intrinsic property of microgels is their ability to swell to larger sizes influencing the bulk properties of suspensions. Recent research has been dedicated to making a range of microgel-forming polymers in order to create responsive multifunctional microgels. The microgel system I study is composed of an amphiphilic polymer called Sepimax Zen, meaning they can act as a stabiliser and a thickener. Skin creams start off as a mixture of immiscible liquids; oil and water. Formulators need to add ingredients which stabilise the mixture and tune the rheology of products. Two metrics by which skin creams are assessed are shelf-life and the performance of the product. The structuring of the formulation determines the shelf-life, while the performance is related to the rheology.
The flow of microgels is an active area of research, particularly with the emergence of multifunctional microgels. Typically, microgel suspensions are viscoelastic materials characterised by ‘non-Newtonian’ flow. The viscosity changes as the flow rate is increased, which is meditated by how individual microgels interact. Sepimax Zen microgels are shear thinning materials, meaning viscosity decreases as flow rate is increased. When concentrated enough, they develop a yield stress, a characteristic of a network forming. Furthermore, Differential Dynamic Microscopy (DDM)  has been an invaluable method for assessing how the size and dynamics of microgels depend on polymer concentration. DDM is an optical technique which can determine the dynamics of the microgels from videos. In dilute suspensions, the size of microgels can be determined, whilst at higher concentrations gel formation can be probed.
Sepimax Zen microgels have a similar rheological profile to the Carbopol, a leading commercially available microgel system. In terms of yield stress and viscosity, both systems give a silky feel and shear down nicely to microscopic films, ideal for topical formulations. In addition to a rheology modifier, formulations also require stabilisers to ensure products stay emulsified. Surface active agents such as surfactants or Pickering stabilisers are added to stop coalescence and sedimentation of emulsion droplets. Sepimax Zen microgels localise at oil-water interfaces, forming closely packed layers.
The next steps of my study will be focussed on building up an understanding of how these microgels stabilise emulsions. Relating the bulk rheological characteristics to the molecular design of the polymers, will provide a guide for tuning the amphiphilicity of new multifunctional polymeric systems. The aim being to establish a model of how surface active microgels can be used to simplify emulsion-based products, leading to reductions in manufacturing and production costs.
 Martinez, Vincent A., et al. "Differential dynamic microscopy: A high-throughput method for characterizing the motility of microorganisms." Biophysical journal 103.8 (2012): 1637-1647.