Grand Challenges in Soft Matter

From the food we cook in the kitchen, to the living cells which form our bodies, Soft Matter is ubiquitous in our every-day lives. How well do we understand Soft Matter and what do we miss to improve our comprehension of this fascinating class of matter? What are the challenges we face to translate fundamental understanding into impactful applications? This short personal perspective tackles some of the challenges associated with Soft Matter and provides possible directions forwards in addressing these challenges. From a chemistry perspective, the field of Soft Matter is just about a century-old, as one could very well consider the 1920 seminal paper of Staudinger “Über Polymerisation,” (Staudinger, 1920) as the spark which set the fire to the explosion of polymer chemistry, one of the core pillars of Soft Matter today. Since then, polymer chemistry has continuously reshaped the landscape of polymers. Polymers are easy to produce and can be cast or moulded into any possible shape; polymer synthesis can be engineered by catalysts to have very low-energy requirements and deliver polymers with high control precision in their molecular architecture. It is for these reasons (and much more) that today “plastics” has become one of the most widely spread man-made materials around the globe, but it is also via advanced polymer chemistry approaches that we are today actively seeking valid solutions for switching from petroleum-based plastics to biodegradable polymers (Tian et al., 2012), to enter into a more sustainable era of polymers, in full harmony with the environment and reducing their global impact on our society. From a physics perspective, the field of Soft Matter as a distinct scientific discipline effectively started only 15 years earlier, with the 1905 annus mirabilis seminal paper of Albert Einstein on Brownian motion (Einstein, 1905). This paper introduces a few ground-breaking concepts over which Soft Matter is still centred today, such as for example the linear t dependence of the mean square displacement of colloidal particles, or the derivation of the expression for the diffusion coefficient of a colloidal particle, an equation to which we today refer by as the Stokes-Einstein law. But perhaps one sentence is particularly revealing in that paper by Einstein: “. . .and is not apparent why a number of suspended particles should not produce the same osmotic pressure as the same number of molecules” (Einstein et al., 1956). The underlying assumption behind this sentence is that, by being the particles “suspended” they must possess an energy of the order of KbT: if not they would either sediment to the bottom or float to the surface of the fluid on which they are suspended. One could actually start from this very same sentence to provide an accurate definition of a colloidal particle, by defining it as any entity with a kinetic energy of the order of KbT, or whose trajectory follows a random walk, for which its mean square displacement acquires a time dependence linear with time t. One is then confronted with a possible first definition of Soft Matter as any material, may this be a liquid, a fluid, a glass or solid, organized by the assembly of building blocks whose energy is on the order of a few KbT. Taking one step further the sentence of Einstein quoted above, let consider a classical particle of radius R and density ρS moving in a fluid of density ρL and viscosity η having reached a terminal steady-state velocity v based on the equilibrium between the forces of gravity (4/3πρSgR 3), buoyancy Edited and Reviewed by: Jay X. Tang, Brown University, United States