We experimentally investigate active motion of spherical Janus colloidal particles in a viscoelastic fluid. Self-propulsion is achieved by a local concentration gradient of a critical polymer mixture which is imposed by laser illumination. Even in the regime where the fluid's viscosity is independent of the deformation rate induced by the particle, we find a remarkable increase of up to 2 orders of magnitude of the rotational diffusion with increasing particle velocity, which can be phenomenologically described by an effective rotational diffusion coefficient dependent on the Weissenberg number. We show that this effect gives rise to a highly anisotropic response of microswimmers in viscoelastic media to external forces, depending on its orientation. DOI: 10.1103/PhysRevLett.116.138301 Nature offers a plethora of microswimmers moving in complex fluid environments [1], whose properties can deviate from Newtonian behavior due to the presence of suspended macromolecules and colloidal particles [2]. Some examples are bacteria in polymeric solutions [3], spermatozoa in cervical mucus [4], and microbial pathogens in stomach mucus [5]. All of these fluids are viscoelastic; i.e., they may exhibit either liquid-or solidlike behavior, depending on imposed deformation rates. Understanding the dynamics of such kinds of microscopic systems is a topic of fundamental significance in statistical mechanics, as they exhibit new types of nonequilibrium processes [6].Despite their biological and application-related relevance, most experiments with autonomous synthetic microswimmers which are self-propelled, e.g., by diffusiophoresis [7][8][9] and thermophoresis [10], were performed in Newtonian fluids [1]. In contrast, only a few studies have considered non-Newtonian fluids [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] where viscoelasticity [11][12][13][15][16][17], shear thinning [20][21][22][23], and shear thickening [20] strongly impact self-propulsion. Previous studies with biological microswimmers suggest that, under such conditions, the dynamical response of the liquid to configurational body changes (e.g., flagellar and undulatory motion) during self-propulsion must be considered [17,18] and can lead to either an increase [11,13,[16][17][18][19]24], a decrease [12,14,17,18,23], or no change [21] of their swimming speed. To avoid specific effects due to such configurational changes and to focus on how the transient strain of viscoelastic fluids couples to the swimmer's stochastic motion, experiments with rigid microswimmers of simple shape are required.In this Letter, we study the motion of artificial microswimmers in a viscoelastic fluid. Their self-propulsion is achieved by light illumination, which allows us to adjust the propulsion velocity. Contrary to Newtonian liquids, we observe a drastic increase of their rotational diffusion with increasing velocity. Such enhancement is independent of whether the velocity is acquired by self-propulsion or by an external force, and it can be quantitatively described...

Autocatalysis is essential for the origin of life and chemical evolution. However, the lack of a unified framework so far prevents a systematic study of autocatalysis. Here, we derive, from basic principles, general stoichiometric conditions for catalysis and autocatalysis in chemical reaction networks. This allows for a classification of minimal autocatalytic motifs called cores. While all known autocatalytic systems indeed contain minimal motifs, the classification also reveals hitherto unidentified motifs. We further examine conditions for kinetic viability of such networks, which depends on the autocatalytic motifs they contain and is notably increased by internal catalytic cycles. Finally, we show how this framework extends the range of conceivable autocatalytic systems, by applying our stoichiometric and kinetic analysis to autocatalysis emerging from coupled compartments. The unified approach to autocatalysis presented in this work lays a foundation toward the building of a systems-level theory of chemical evolution.