We study the dynamics and conformation of polymers composed by active monomers. By means of Brownian dynamics simulations we show that when the direction of the self-propulsion of each monomer is aligned with the backbone, the polymer undergoes a coil-to-globule-like transition, highlighted by a marked change of the scaling exponent of the gyration radius. Concurrently, the diffusion coefficient of the center of mass of the polymer becomes essentially independent of the polymer size for sufficiently long polymers or large magnitudes of the self-propulsion. These effects are reduced when the self-propulsion of the monomers is not bound to be tangent to the backbone of the polymer. Our results, rationalized by a minimal stochastic model, open new routes for activity-controlled polymer and, possibly, for a new generation of polymer-based drug carriers.

We show that when particles are suspended in an electrolyte confined between corrugated charged surfaces, electrokinetic flows lead to a new set of phenomena such as particle separation, mixing for low-Reynolds micro-and nano-metric devices and negative mobility. Our analysis shows that such phenomena arise, for incompressible fluids, due to the interplay between the electrostatic double layer and the corrugated geometrical confinement and that they are magnified when the width of the channel is comparable to the Debye length. Our characterization allows us to understand the physical origin of such phenomena therefore shading light on their possible relevance in a wide variety of situations, ranging from nano-and micro-fluidic devices to biological systems.PACS numbers: 82.39. Wj,47.56.+r,47.61.Fg The recent development of nano-and micro-fluidic devices [1] as well as cellular regulation mechanisms and cellular signaling [2] rely on the transport of ions across channels or pores whose sections range from the nanometric to the micrometric scale [3][4][5]. The transport across such conduits has been characterized, even for varying-section channels [6][7][8][9][10][11], assuming that the channel width, h(x), is large compared to the Debye length, κ −1 , over which the electrolyte charge distributes in the neighborhood of the charged channel wall (κh(x) ≫ 1), or in the absence of electrolytes [12]. Nowadays, the continuous process of device miniaturization and the widening of the range of achievable salt concentrations require an understanding of the behavior of such systems when the relevant length scales compete with each other [13]. Such regimes are already exploitable in different microand nano-fluidic experiments [3,4] and can be relevant in a variety of biological systems [14,15].In this Letter we will show that, precisely in this regime, i.e. when the Debye length and the channel aperture are comparable in size, κh(x) ∼ 1, an electrolyte embedded in a corrugated channel develops new transport regimes that can be exploited to separate suspended particles, control electric and mass currents and eventually induce negative mobility. When κh(x) ∼ 1, the electrolyte response to external forcing, such as electrostatic fields, is very sensitive to the channel shape and it develops a recirculating region in which the electrolyte flows on the opposite direction as compared to the average volume flow, as shown in Fig. 1.A. Such a phenomenon, typical for incompressible fluids, is due to the interplay between the electrostatic double layer and the varying geometrical confinement and its magnitude is significantly amplified when κh(x) ∼ 1. We coin this regime entropic electrokinetics since the phenomena we identify can only arise due to the spatially varying constriction induced by the geometrical confinement. This variation affects the local spatial distribution of ions, essentially controlled by the interplay between the wall charge and the ion entropy. We will show that the entropic variations in the charge density i...

We show theoretically that near a fluid-fluid interface a single active colloidal particle generating, e.g., chemicals or a temperature gradient experiences an effective force of hydrodynamic origin. This force is due to the fluid flow driven by Marangoni stresses induced by the activity of the particle; it decays very slowly with the distance from the interface, and can be attractive or repulsive depending on how the activity modifies the surface tension. We show that, for typical systems, this interaction can dominate the dynamics of the particle as compared to Brownian motion, dispersion forces, or self-phoretic effects. In the attractive case, the interaction promotes the self-assembly of particles into a crystal-like monolayer at the interface. DOI: 10.1103/PhysRevLett.116.078301 Significant attention has been paid lately to micrometer sized particles capable of self-induced motility [1][2][3]. They are seen as promising candidates for novel techniques in chemical sensing [4] or water treatment [5]. The motion of active colloidal particles has been the subject of numerous experimental [1][2][3]6,7] and theoretical [8][9][10][11][12] studies. One realization is a particle with a catalytic surface promoting a chemical reaction in the surrounding solution [13]. For an axisymmetric particle lacking fore-and-aft symmetry, the distributions of reactant and product molecules may become nonuniform along its surface and the particle could move due to self-induced phoresis [14]. If the particle is spherically symmetric, it will remain immobile in bulk solution but can be set into motion by the vicinity of walls or other particles (not necessarily active) which break the spherical symmetry [10,[13][14][15][16].A relevant case corresponds to the movement of active particles bounded by a fluid-fluid interface. This situation raises new issues, in particular if the reactants or the products have a significant effect on the properties of the fluid interface implying tensioactivity. For example, it has been recently predicted that catalytically active, spherical particles which are trapped at the interface may be set into motion along the interface by Marangoni flows, selfinduced via the spatially nonuniform distribution of tensioactive molecules [17][18][19]. (A similar motility mechanism can originate from thermally induced Marangoni flows if, e.g., the particle contains a metal cap which is heated by a laser beam [20].) Furthermore, self-induced Marangoni flows, combined with a mechanism of triggering spontaneous symmetry breaking, have also been used to develop self-propelled droplets [21][22][23].However, another category of experimental situations occurs if the particles are not trapped at the interface but may reside in the vicinity of the interface or get near it during their motion. In this study we provide theoretical evidence that such catalytically active or locally heated spherical particles, although immobile in bulk, experience a very strong, long-ranged effective force field due to the Marangoni stresses...