Structure, heat, elasticity, and their mutual interplay across different length scales in soft polymeric composite materials – Final report DFG Heisenberg Program

Soft matter is characterized by its relatively strong response to external and internal influences. A soft piece of rubber or biological tissue can be easily deformed between our fingers using muscle force, whereas a steel rod does not show visible deformations detectable by eye. We use theoretical calculations and computer simulations to investigate the properties of soft matter when it has additional functionalities. These can include magnetic or living components. We are particularly interested in how individual microscopic components contribute to the behavior and properties of the entire system. In the first case, we examine rubbers (elastomers) containing microscopic particles that become magnetized in an external magnetic field. Due to resulting magnetic forces between individual particles, the entire material can deform and change its stiffness. This allows deformations to be generated magnetically or the damping behavior of the materials to be externally adjusted, even during use. We have derived a mathematical relationship between the positions of the individual particles and the resulting overall deformation of the material, such as changes in volume or expansion along the external magnetic field. In a key step, we identified through an optimization process particle arrangements that lead to maximum deformations and maximum hardening or softening of the materials. As a second example, we investigate so-called active matter. It includes microscopic objects that move autonomously. We can think of microorganisms like bacteria in a liquid or in a soft solid. Viscoelastic media, which can both flow and exhibit temporary elastic responses, are likewise considered as enclosing materials. Interestingly, depending on the properties of the environment, the active particles can move in a disordered manner, collectively in one direction, organize into swarms with turbulent movement patterns, move in stripe patterns, or perform collective circular motions. Special flow properties of a carrier fluid can lead to regular structures. We have predicted the existence of several of these dynamic states. Many of these phenomena were derived for thin layers and films. We were able to generally expand the understanding of membrane elasticity and flow behavior in films, particularly regarding movements within the film plane. Additional examples of our work include the growth and spreading of bent bacteria on surfaces. In general, we will continue to work on how properties of artificially activated materials and active, living matter can be described and potentially complement each other.