Resolving the Rub in Food Friction (MSc Raisa Rudge)

The science of friction, lubrication and wear, commonly referred to as tribology, has been explored for centuries. Friction plays a role in applications in a broad range of different fields including foods, biomedical, mechanical and automotive engineering as evidently reducing friction means reducing material wear, costs and energy savings. The field of tribology started with pioneers including Leonardo da Vinci and Guillaume Amontons. Centuries later, we have a reasonable understanding of dry friction on hard materials, which is well described by Amontons’ frictional laws.

There is however much work to do on understanding the physics and physical chemistry behind the frictional between soft and/or fluid-immersed solids such as hydrogels. Friction on soft and wet substrates is of increasing interest in several fields, including the food and biomedical industry. Hydrogels are a good model system to study friction, as they are soft, porous materials made of (bio)polymers that are able to hold large amounts of water. Interactions between hydrogels are easily tuneable as they depend on chemical composition, pH, temperature etc. The friction behind these soft materials is thus much more complex than solid friction which gives opportunities to both develop them for new applications and to use them to uncover fundamental frictional mechanisms. Interestingly, hydrogels have already been found to typically experience very little frictional dissipation, as anyone who has tried to pick up a wet gummy bear can attest. This makes hydrogels concretely relevant for application such as artificial joints, and will allow to make food materials with improved texture and structure. Furthermore quantifying the lubrication properties taking place in the mouth while eating semi-solid foods can help eliminate sensory panels, saving time and money.  

For all these reasons it is fundamentally interesting to study hydrogel-hydrogel friction. The fundamental question in gel friction is: which types of dissipation channels play a role in the friction between watery gel-like materials? Using different tribological methods including a rheometer equipped with a tribotool designed by our group, we aim at unravelling the complex mechanism behind hydrogel-hydrogel friction. Microscopic techniques such as cryo-SEM and optical tweezers are used to further evaluate interactions at microscopic level. Additionally, we apply our knowledge on macroscopic hydrogel friction to design hydrogel suspensions with novel frictional properties. Further investigating the complex frictional behavior of hydrogels can aid in selecting and developing new food materials or innovative medical devices.