The Mere Categorization Effect: How the Presence of Categories Increases Choosers' Perceptions of Assortment Variety

What is the effect of option categorization on choosers’ satisfaction? A combination of field and laboratory experiments reveals that the mere presence of categories, irrespective of their content, positively influences the satisfaction of choosers who are unfamiliar with the choice domain. This “mere categorization effect” is driven by a greater number of categories signaling greater variety amongst the available options, which allows for a sense of self-determination from choice. This effect, however, is attenuated among choosers who are familiar with the choice domain, who do not rely on the presence of categories to perceive the variety available.

Soft Listeria: actin-based propulsion of liquid drops

We study the motion of oil drops propelled by actin polymerization in cell extracts. Drops deform and acquire a pear-like shape under the action of the elastic stresses exerted by the actin comet. We solve this free boundary problem and calculate the drop shape taking into account the elasticity of the actin gel and the variation of the polymerization velocity with normal stress. The pressure balance on the liquid drop imposes a zero propulsive force if gradients in surface tension or internal pressure are not taken into account. Quantitative parameters of actin polymerization are obtained by fitting theory to experiment.Comment: 5 pages, 4 figure

Dynamic coordinated control laws in multiple agent models

We present an active control scheme of a kinetic model of swarming. It has been shown previously that the global control scheme for the model, presented in \cite{JK04}, gives rise to spontaneous collective organization of agents into a unified coherent swarm, via a long-range attractive and short-range repulsive potential. We extend these results by presenting control laws whereby a single swarm is broken into independently functioning subswarm clusters. The transition between one coordinated swarm and multiple clustered subswarms is managed simply with a homotopy parameter. Additionally, we present as an alternate formulation, a local control law for the same model, which implements dynamic barrier avoidance behavior, and in which swarm coherence emerges spontaneously.Comment: 20 pages, 6 figure

Dynamics of an inchworm nano-walker

An inchworm processive mechanism is proposed to explain the motion of dimeric molecular motors such as kinesin. We present here preliminary results for this mechanism focusing on observables like mean velocity, coupling ratio and efficiency versus ATP concentration and the external load F.Comment: 6 pages, 2 figure

Symmetry-Breaking Motility

Locomotion of bacteria by actin polymerization, and in vitro motion of spherical beads coated with a protein catalyzing polymerization, are examples of active motility. Starting from a simple model of forces locally normal to the surface of a bead, we construct a phenomenological equation for its motion. The singularities at a continuous transition between moving and stationary beads are shown to be related to the symmetries of its shape. Universal features of the phase behavior are calculated analytically and confirmed by simulations. Fluctuations in velocity are shown to be generically non-Maxwellian and correlated to the shape of the bead.Comment: 4 pages, 2 figures, REVTeX; formatting of references correcte

Branching, Capping, and Severing in Dynamic Actin Structures

Branched actin networks at the leading edge of a crawling cell evolve via protein-regulated processes such as polymerization, depolymerization, capping, branching, and severing. A formulation of these processes is presented and analyzed to study steady-state network morphology. In bulk, we identify several scaling regimes in severing and branching protein concentrations and find that the coupling between severing and branching is optimally exploited for conditions {\it in vivo}. Near the leading edge, we find qualitative agreement with the {\it in vivo} morphology.Comment: 4 pages, 2 figure

Time, Money, and Morality

Money, a resource that absorbs much daily attention, seems to be present in much unethical behavior thereby suggesting that money itself may corrupt. This research examines a way to offset such potentially deleterious effects—by focusing on time, a resource that tends to receive less attention than money but is equally ubiquitous in our daily lives. Across four experiments, we examine whether shifting focus onto time can salvage individuals' ethicality. We found that implicitly activating the construct of time, rather than money, leads individuals to behave more ethically by cheating less. We further found that priming time reduces cheating by making people reflect on who they are. Implications for the use of time versus money primes in discouraging or promoting dishonesty are discussed

The Force-Velocity Relation for Growing Biopolymers

The process of force generation by the growth of biopolymers is simulated via a Langevin-dynamics approach. The interaction forces are taken to have simple forms that favor the growth of straight fibers from solution. The force-velocity relation is obtained from the simulations for two versions of the monomer-monomer force field. It is found that the growth rate drops off more rapidly with applied force than expected from the simplest theories based on thermal motion of the obstacle. The discrepancies amount to a factor of three or more when the applied force exceeds 2.5kT/a, where a is the step size for the polymer growth. These results are explained on the basis of restricted diffusion of monomers near the fiber tip. It is also found that the mobility of the obstacle has little effect on the growth rate, over a broad range.Comment: Latex source, 9 postscript figures, uses psfig.st

Caracterización y biocompatibilidad de matrices de colágeno para uso en regeneración ósea

El colágeno, la proteína más abundante del hueso, juega un rol fundamental en la integridad biológica y estructural del esqueleto. Previamente se han usado membranas de colageno sin un orden molecular, para fabricar matrices para la regeneración del tejido óseo. El colágeno es así un candidato natural para mejorar o reemplazar tejidos u órganos dañados. El objetivo del presente trabajo es caracterizar matrices de colágeno ordenado o no (con una distribución al azar) y estudiar su biocompatiblidad con células óseas en cultivo. Se estilizó colágeno extraído del tendón de Aquiles bovino, nativo, obtenido en nuestro laboratorio con un grado de pureza de un 98% [Ruderman et al., 2007]. Se fabricaron matrices de colágeno no ordenado y de colágeno ordenado según patentes. Las características de la superficie de membranas fueron observadas por SEM y microscopia óptica (coloración de Sirius red). Las membranas ordenadas mostraron una topografía típica en forma de canales en correlación con un ordenamiento molecular. Se evaluó la biocompatibilidad de células osteoblásticas y macrófagos murinos crecidos sobre los dos tipos de películas de colágeno (No ordenado y ordenado). Se estudió la adhesión, proliferación (conteo de células teñidas con Giemsa) y diferenciación al fenotipo osteoblasto (expresión de fosfatasa alcalina y nódulos de mineralización). Se encontró que las células (osteoblasticas y macrófagos) crecidas sobre las matrices de colágeno ordenado se adhieren mas (1.5-1.7 veces) y crecen mejor (2.3–2.6 veces) que sobre las matrices de colágeno no ordenado. Macrófagos Raw 264.7 teñidos con naranja de acridina revelaron mayor cantidad de células muertas sobra las matrices de colágeno no ordenado. Preosteoblástos MC3T3E1 (4 semanas en medio osteogénico) expresaron más fosfatasa alcalina (2.6 veces) y mineral en la matriz de colágeno. Los estudios preliminares sugieren que las matrices preparadas en base a colágeno natural podrían ser aplicadas en la regeneración del tejido

Single cell mechanics: stress stiffening and kinematic hardening

Cell mechanical properties are fundamental to the organism but remain poorly understood. We report a comprehensive phenomenological framework for the nonlinear rheology of single fibroblast cells: a superposition of elastic stiffening and viscoplastic kinematic hardening. Our results show, that in spite of cell complexity its mechanical properties can be cast into simple, well-defined rules, which provide mechanical cell strength and robustness via control of crosslink slippage.Comment: 4 pages, 6 figure