*“Science is the belief in the ignorance of experts” (R. Feynman)*

###### How Activity Landscapes Polarize Microswimmers without Alignment Forces

*in Phys. Rev. Lett. 2021*

*by N. A. Söker, S. Auschra, V. Holubec, K. Kroy, and F. Cichos*

Active-particle suspensions exhibit distinct polarization-density patterns in activity landscapes, even without anisotropic particle interactions. Such polarization without alignment forces is at work in motility-induced phase separation and betrays intrinsic microscopic activity to mesoscale observers. Using stable long-term confinement of a single thermophoretic microswimmer in a dedicated force-free particle trap, we examine the polarized interfacial layer at a motility step and confirm that it does not exert pressure onto the bulk. Our observations are quantitatively explained by an analytical theory that can also guide the analysis of more complex geometries and many-body effects.

###### Active Brownian Heat Engine

*in Phys. Rev. Res. 2020*

*by V. Holubec, S. Steffenoni, G. Falasco, & K. Kroy*

When do nonequilibrium forms of disordered energy qualify as heat? We address this question in the context of cyclically operating heat engines feeding on nonequilibrium energy reservoirs that defy the zeroth law of thermodynamics into work. To consistently address a nonequilibrium bath as a heat bath in the sense of the second law of thermodynamics requires the existence of a precise mapping to an equivalent cycle with an equilibrium bath at a time-dependent effective temperature. We identify the most general setup for which this can generically be ascertained and thoroughly discuss an analytically tractable, experimentally relevant scenario: a Brownian particle confined in a periodically modulated harmonic potential and coupled to some nonequilibrium bath of variable activity.

###### Physically consistent numerical solver for time-dependent Fokker-Planck equations

*in Phys. Rev. E 2019*

*by V. Holubec, K. Kroy, & S. Steffenoni *

We present a simple thermodynamically consistent method for solving time-dependent Fokker-Planck equations (FPE) for overdamped stochastic processes, also known as Smoluchowski equations. It yields both transition and steady-state behavior and allows for computations of moment-generating and large-deviation functions of observables defined along stochastic trajectories, such as the fluctuating particle current, heat, and work. The key strategy is to approximate the FPE by a master equation with transition rates in configuration space that obey a local detailed balance condition for arbitrary discretization. Its time-dependent solution is obtained by a direct computation of the time-ordered exponential, representing the propagator of the FPE, by summing over all possible paths in the discretized space. The method thus not only preserves positivity and normalization of the solutions but also yields a physically reasonable total entropy production, regardless of the discretization.

###### Aeolian sand sorting and megaripple formation

*in Nature Physics 2018*

*by M. Lämmel, A. Meiwald, H. Yizhaq, H. Tsoar, I. Katra & K. Kroy*

Sand is blown across beaches and deserts by turbulent winds. This seemingly chaotic process creates two dominant bedforms: decametre-scale dunes and centimetre-scale ripples, but hardly anything in between. By the very same process, grains are constantly sorted. Smaller grains advance faster, while heavier grains trail behind. Here, we argue that, under erosive conditions, sand sorting and structure formation can conspire to create distinct bedforms in the ‘forbidden wavelength gap’ between aeolian ripples and dunes. These so-called megaripples are shown to co-evolve with an unusual, predominantly bimodal grain-size distribution. Combining theory and field measurements, we develop a mechanistic understanding of their formation, shape and migration, as well as their cyclic ageing, renewal and sedimentary memory, in terms of the intermittent wind statistics. Our results demonstrate that megaripples exhibit close similarities to dunes and can indeed be mechanistically characterized as a special type of (‘reptation’) dune.

see also Physics World, Physics Today, PhysOrg News, Press Release