Tracer dynamics in polymer networks: Generalized Langevin description.

Tracer diffusion in polymer networks and hydrogels is relevant in biology and technology, while it also constitutes an interesting model process for the dynamics of molecules in fluctuating, heterogeneous soft matter. Here, we systematically study the time-dependent dynamics and (non-Markovian) memory effects of tracers in polymer networks based on (Markovian) implicit-solvent Langevin simulations. In particular, we consider spherical tracer solutes at high dilution in regular, tetrafunctional bead-spring polymer networks and control the tracer-network Lennard-Jones (LJ) interactions and the polymer density. Based on the analysis of the memory (friction) kernels, we recover the expected long-time transport coefficients and demonstrate how the short-time tracer dynamics, polymer fluctuations, and the viscoelastic response are interlinked. Furthermore, we fit the characteristic memory modes of the tracers with damped harmonic oscillations and identify LJ contributions, bond vibrations, and slow network relaxations. Tuned by the LJ interaction parameter, these modes enter the kernel with an approximately linear to quadratic scaling, which we incorporate into a reduced functional form for convenient tracer memory interpolation and extrapolation. This eventually leads to highly efficient simulations utilizing the generalized Langevin equation, in which the polymer network acts as an additional thermal bath with a tunable intensity.

Generalized Langevin dynamics simulation with non-stationary memory kernels: How to make noise.

We present a numerical method to produce stochastic dynamics according to the generalized Langevin equation with a non-stationary memory kernel. This type of dynamics occurs when a microscopic system with an explicitly time-dependent Liouvillian is coarse-grained by means of a projection operator formalism. We show how to replace the deterministic fluctuating force in the generalized Langevin equation by a stochastic process, such that the distributions of the observables are reproduced up to moments of a given order. Thus, in combination with a method to extract the memory kernel from simulation data of the underlying microscopic model, the method introduced here allows us to construct and simulate a coarse-grained model for a driven process.

Alterations in the Experience of Time, Space, and Intersubjectivity and the Interaction with Pre-Existent Psychopathology during the COVID-19 Pandemic.

BACKGROUND: The COVID-19 pandemic and the measures to protect the physically vulnerable may disproportionately affect people with mental health vulnerabilities, who receive psychotherapeutic inpatient treatment, as many of these measures impact the (inter)subjective space crucial to psychotherapy. OBJECTIVE: We investigate how people with pre-existing mental health conditions and healthcare professionals experienced changes linked to the COVID-19 pandemic. METHODS: During the first COVID-19 outbreak and lockdown in spring 2020, we conducted semi-structured interviews with patients and healthcare professionals at a clinic for psychosomatic medicine in Germany and analyzed them following the principles of descriptive phenomenology focusing on social interactions, intersubjectivity, and the therapeutic space. RESULTS: We conducted a total of >30 h of interviews with 19 patients and 17 healthcare professionals. Analyses revealed that the COVID-19 pandemic and the related measures have led to a sudden loss of a sense of normality. Participants experienced changes in the perception of time, space, self, and embodied interaction with others, resulting in a profound feeling of alienation and "unhomeliness" which seemed to magnify pre-existing psychopathology. The inpatient psychotherapeutic environment provided safety by offering spatial and temporal structures and opportunities for social interaction, supporting people to find new ways to be in a changed world. CONCLUSIONS: Our study shows that despite the threat of infection, it is vital to continue to provide people with psychological vulnerabilities with a safe therapeutic space in which to regain a sense of safety in a changed world. This is particularly important, as those people seem to suffer intensely from the collateral measures of a pandemic.