Rafael Delgado-Buscalioni

Universidad Autónoma de Madrid, Dept. Theoretical Condensed Matter Physics and Institute for Condensded Matter (IFIMAC)
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Learning from vibrations: unveiling molecular viscoelasticity in hydrodynamic environments

Conservation and dissipation are the Ying and Yang of the dynamic description of any natural system. The conservative part is fully encoded in the free energy of the variables chosen to describe the system and it can be sampled using static perturbations (e.g. AFM indentation). By contrast, the dissipative part can only be measured using dynamic perturbations and the simplest case corresponds to low amplitude monochromatic oscillatory forcing. This idea is used in a myriad of techniques from atoms (Raman) to micron-size structures (AFM vibrational spectra). In these dynamics, conservative and dissipative (or elastic and viscous) responses are proportional to the deformation and the deformation rate, respectively. When samples are moving in a liquid, the problem of determining their intrinsic viscoelasticity substantially complicates. The solvent hydrodynamic substantially distorts the experimental signals adding extra « dissipation ». Experimental signals just provide the response of a strongly coarse-grained variable of the whole system. And to complicate things, thermal noise acts as an added forcing which depends on the variable (via fluctuation dissipation balance). This talk will explore this problem and present routes to extract the intrinsic viscoelasticity of molecules and soft structures from several types of experiments (quartz crystal microbalance, vibrational spectra using AFM, and oscillatory magnetometers). To that end, we will need to computationally solve the coupling between soft matter and oscillatory hydrodynamics [1] and deploy finer-detailed viscoelastic versions of the molecular samples. As the Ying and Yang, elastic and viscous response are linked: mathematically by Kramers-Kronig and physically by Mori-Zwanzig theories. How can we transfer experimental information to finer scales? what is consistency between scales? what is limit of such approach? While I do not have all the answers, I will offer you these questions.

[1]R. Peláez, P. Palacios-Alonso, R. Delgado-Buscalioni, J. Fluid Mech., 1010, A57 (2025)