Contact forces in microgel suspensions

What is it about?

Within a model where micrometer-size soft colloidal particles are viewed as liquid drops, Vrban, Šiber and Ziherl theoretically study the contact interaction between them. The numerically exact deformation energy is calculated across a broad range of indentations and for various model parameters, and they show that it can be reproduced using either truncated superball or truncated spheropolyhedral variational shapes depending on the parameters. At large surface tensions representative of microgels, this energy is pairwise additive well beyond small indentations and can be approximated by a power-law dependence on indentation with an exponent around 2.

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Photo by Dynamic Wang on Unsplash

Photo by Dynamic Wang on Unsplash

Why is it important?

Microgels are microscopic soft polymeric particles used in a wide range of applications from drug delivery and cosmetics to foodstuffs and oil recovery. Despite their importance across various industries, their mechanical properties remain poorly understood. When pressed against one another, microgels deform, flatten, and partially interpenetrate, resulting in a characteristic contact interaction energy. We develop an effective theory of this interaction by modeling microgels as liquid like droplets with sharp interfaces, analogous to the surfaces of raindrops. An advantage of this approach is that it is based on intuitive and readily visualizable quantities, such as surface area and volume. We use this theory to numerically compute the interaction energy of arrays of microgels, showing that in repulsive microgels this energy is approximately proportional to the square of indentation – much like the energy stored in a metal spring. We further show that the exact shapes of deformed microgels can be approximated rather well using fairly straightforward geometrical forms, which simplifies the theory. Importantly, our theory also distinguishes between compressible and incompressible microgels as well as between attractive and repulsive ones so that it can be employed to describe various types of microgel-based materials.