[ { "id": "authors:3ywyj-92q85", "collection": "authors", "collection_id": "3ywyj-92q85", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180524-132438946", "type": "article", "title": "Anisotropic swim stress in active matter with nematic order", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "Active Brownian particles (ABPs) transmit a swim pressure II^(swim) = n \u03b6D^(swim) to the container boundaries, where \u03b6 is the drag coefficient, D^(swim) is the swim diffusivity and n is the uniform bulk number density far from the container walls. In this work we extend the notion of the isotropic swim pressure to the anisotropic tensorial swim stress \u03c3^(swim) = - n \u03b6D^(swim), which is related to the anisotropic swim diffusivity D^(swim). We demonstrate this relationship with ABPs that achieve nematic orientational order via a bulk external field. The anisotropic swim stress is obtained analytically for dilute ABPs in both 2D and 3D systems. The anisotropy, defined as the ratio of the maximum to the minimum of the three principal stresses, is shown to grow exponentially with the strength of the external field. We verify that the normal component of the anisotropic swim stress applies a pressure II^(swim) = -( \u03c3^(swim) \u00b7 n) \u00b7 n on a wall with normal vector n, and, through Brownian dynamics simulations, this pressure is shown to be the force per unit area transmitted by the active particles. Since ABPs have no friction with a wall, the difference between the normal and tangential stress components\u2014the normal stress difference\u2014generates a net flow of ABPs along the wall, which is a generic property of active matter systems.", "doi": "10.1088/1367-2630/aac3c5", "issn": "1367-2630", "publisher": "IOP", "publication": "New Journal of Physics", "publication_date": "2018-05", "series_number": "5", "volume": "20", "issue": "5", "pages": "Art. No. 053056" }, { "id": "authors:nkhqs-gvw15", "collection": "authors", "collection_id": "nkhqs-gvw15", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171201-130807253", "type": "article", "title": "The curved kinetic boundary layer of active matter", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "A body submerged in active matter feels the swim pressure through a kinetic accumulation boundary layer on its surface. The boundary layer results from a balance between translational diffusion and advective swimming and occurs on the microscopic length scale \u03bb^(-1) = \u03b4/\u221a2[1+1/6(\u2113/\u03b4)^2]\n. Here \u03bb = \u221aD_T\u03c4_R, D_T is the Brownian translational diffusivity, \u03c4_R is the reorientation time and \u2113 = U0\u03c4R is the swimmer's run length, with U_0 the swim speed [Yan and Brady, J. Fluid. Mech., 2015, 785, R1]. In this work we analyze the swim pressure on arbitrary shaped bodies by including the effect of local shape curvature in the kinetic boundary layer. When \u03b4 \u226a L and \u2113 \u226a L, where L is the body size, the leading order effects of curvature on the swim pressure are found analytically to scale as JS\u03bb\u03b4^2/L, where JS is twice the (non-dimensional) mean curvature. Particle-tracking simulations and direct solutions to the Smoluchowski equation governing the probability distribution of the active particles show that \u03bb\u03b4^2/L is a universal scaling parameter not limited to the regime \u03b4, \u2113 \u226a L. The net force exerted on the body by the swimmers is found to scale as F^(net)/(n^\u221ek_sT_sL^2) = f(\u03bb\u03b4^2/L), where f(x) is a dimensionless function that is quadratic when x \u226a 1 and linear when x \u223c 1. Here, k_sT_s= \u03b6U0^2\u03c4_R/6 defines the 'activity' of the swimmers, with \u03b6 the drag coefficient, and n^\u221e is the uniform number density of swimmers far from the body. We discuss the connection of this boundary layer to continuum mechanical descriptions of active matter and briefly present how to include hydrodynamics into this purely kinetic study.", "doi": "10.1039/C7SM01643C", "issn": "1744-683X", "publisher": "Royal Society of Chemistry", "publication": "Soft Matter", "publication_date": "2018-01-14", "series_number": "2", "volume": "14", "issue": "2", "pages": "279-290" }, { "id": "authors:908gw-y4w25", "collection": "authors", "collection_id": "908gw-y4w25", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171201-095649069", "type": "article", "title": "Antiswarming: Structure and dynamics of repulsive chemically active particles", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "Chemically active Brownian particles with surface catalytic reactions may repel each other due to diffusiophoretic interactions in the reaction and product concentration fields. The system behavior can be described by a \"chemical\" coupling parameter \u0393_c that compares the strength of diffusiophoretic repulsion to Brownian motion, and by a mapping to the classical electrostatic one component plasma (OCP) system. When confined to a constant-volume domain, body-centered cubic (bcc) crystals spontaneously form from random initial configurations when the repulsion is strong enough to overcome Brownian motion. Face-centered cubic (fcc) crystals may also be stable. The \"melting point\" of the \"liquid-to-crystal transition\" occurs at \u0393_c \u2248 140 for both bcc and fcc lattices.", "doi": "10.1103/PhysRevE.96.060601", "issn": "2470-0045", "publisher": "American Physical Society", "publication": "Physical Review E", "publication_date": "2017-12", "series_number": "6", "volume": "96", "issue": "6", "pages": "Art. No. 060601" }, { "id": "authors:3yzaz-tx156", "collection": "authors", "collection_id": "3yzaz-tx156", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20161005-090742307", "type": "article", "title": "The behavior of active diffusiophoretic suspensions: An accelerated Laplacian dynamics study", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "Diffusiophoresis is the process by which a colloidal particle moves in response to the concentration gradient of a chemical solute. Chemically active particles generate solute concentration gradients via surface chemical reactions which can result in their own motion \u2014 the self-diffusiophoresis of Janus particles \u2014 and in the motion of other nearby particles \u2014 normal down-gradient diffusiophoresis. The long-range nature of the concentration disturbance created by a reactive particle results in strong interactions among particles and can lead to the formation of clusters and even coexisting dense and dilute regions often seen in active matter systems. In this work, we present a general method to determine the many-particle solute concentration field allowing the dynamic simulation of the motion of thousands of reactive particles. With the simulation method, we first clarify and demonstrate the notion of \"chemical screening,\" whereby the long-ranged interactions become exponentially screened, which is essential for otherwise diffusiophoretic suspensions would be unconditionally unstable. Simulations show that uniformly reactive particles, which do not self-propel, form loosely packed clusters but no coexistence is observed. The simulations also reveal that there is a stability threshold \u2014 when the \"chemical fuel\" concentration is low enough, thermal Brownian motion is able to overcome diffusiophoretic attraction. Janus particles that self-propel show coexistence, but, interestingly, the stability threshold for clustering is not affected by the self-motion.", "doi": "10.1063/1.4963722", "issn": "0021-9606", "publisher": "American Institute of Physics", "publication": "Journal of Chemical Physics", "publication_date": "2016-10-07", "series_number": "13", "volume": "145", "issue": "13", "pages": "Art. No. 134902" }, { "id": "authors:5vae3-1bt47", "collection": "authors", "collection_id": "5vae3-1bt47", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160328-102204704", "type": "article", "title": "The force on a boundary in active matter", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "We present a general theory for determining the force (and torque) exerted on a boundary (or body) in active matter. The theory extends the description of passive Brownian colloids to self-propelled active particles and applies for all ratios of the thermal energy k_BT to the swimmer's activity k_sT_s=\u03b6U^2_0\u03c4_R/6, where \u03b6 is the Stokes drag coefficient, U0_ is the swim speed and \u03c4_R is the reorientation time of the active particles. The theory, which is valid on all length and time scales, has a natural microscopic length scale over which concentration and orientation distributions are confined near boundaries, but the microscopic length does not appear in the force. The swim pressure emerges naturally and dominates the behaviour when the boundary size is large compared to the swimmer's run length \u2113=U_0\u03c4_R. The theory is used to predict the motion of bodies of all sizes immersed in active matter.", "doi": "10.1017/jfm.2015.621", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2015-12", "volume": "785", "pages": "Art. No. R1" }, { "id": "authors:yahyc-r9794", "collection": "authors", "collection_id": "yahyc-r9794", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150714-134052439", "type": "article", "title": "The swim force as a body force", "author": [ { "family_name": "Yan", "given_name": "Wen", "orcid": "0000-0002-9189-0840", "clpid": "Yan-Wen" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "Net (as opposed to random) motion of active matter results from an average swim (or propulsive) force. It is shown that the average swim force acts like a body force \u2013 an internal body force. As a result, the particle-pressure exerted on a container wall is the sum of the swim pressure [Takatori et al., Phys. Rev. Lett., 2014, 113, 028103] and the 'weight' of the active particles. A continuum description is possible when variations occur on scales larger than the run length of the active particles and gives a Boltzmann-like distribution from a balance of the swim force and the swim pressure. Active particles may also display 'action at a distance' and accumulate adjacent to (or be depleted from) a boundary without any external forces. In the momentum balance for the suspension \u2013 the mixture of active particles plus fluid \u2013 only external body forces appear.", "doi": "10.1039/C5SM01318F", "issn": "1744-683X", "publisher": "Royal Society of Chemistry", "publication": "Soft Matter", "publication_date": "2015-08-21", "series_number": "31", "volume": "11", "issue": "31", "pages": "6235-6244" } ]