Collective Behavior

Nanoparticles of CdTe were found to spontaneously reorganize into crystalline nanowires upon controlled removal of the protective shell of organic stabilizer. The intermediate step in the nanowire formation was found to be pearl-necklace aggregates. Strong dipole-dipole interaction is believed to be the driving force of nanoparticle self-organization. The linear aggregates subsequently recrystallized into nanowires whose diameter was determined by the

Experiments and mathematical modeling show that complex flows driven by unexpected flagellar arrangements are induced when peritrichously flagellated bacteria are confined in a thin layer of fluid, between asymmetric boundaries. The flagella apparently form a dynamic bipolar assembly rather than the single bundle characteristic of free swimming bacteria, and the resulting flow is observed to circulate around the cell body. It ranges over several cell diameters, in contrast to the small extent of the flows surrounding free swimmers. Results also suggest that flagellar bundles on bacteria that lie flat on a solid substrate have an effective rotation rate slower than “free” flagella. This discovery extends our knowledge of the dynamic geometry of bacteria and their flagella, and reveals new mechanisms for motility-associated molecular transport and intercellular communication.

Novel aspects of human dynamics and social interactions are investigated by means of mobile phone data. Using extensive phone records resolved in both time and space, we study the mean collective behavior at large scales and focus on the occurrence of anomalous events. We discuss how these spatiotemporal anomalies can be described using standard percolation theory tools. We also investigate patterns of calling activity at the individual level and show that the interevent time of consecutive calls is heavy-tailed. This finding, which has implications for dynamics of spreading phenomena in social networks, agrees with results previously reported on other human activities.

Aerobic bacteria often live in thin fluid layers near solid–air–water contact lines, in which the biology of chemotaxis, metabolism, and cell–cell signaling is intimately connected to the physics of buoyancy, diffusion, and mixing. Using the geometry of a sessile drop, we demonstrate in suspensions of Bacillus subtilis the self-organized generation of a persistent hydrodynamic vortex that traps cells near the contact line. Arising from upward oxygentaxis and downward gravitational forcing, these dynamics are related to the Boycott effect in sedimentation and are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics. The vortex is shown to advectively enhance uptake of oxygen into the suspension, and the wedge geometry leads to a singularity in the chemotactic dynamics near the contact line.

We describe experiments that show collective behavior in clouds of optically trapped neutral atoms. This collective behavior is demonstrated in a variety of observed spatial distributions with abrupt bistable transitions between them. These distributions include stable rings of ...

The field of nonlinear “induced-charge” electrokinetics is rapidly advancing, motivated by potential applications in microfluidics as well as by the unique opportunities it provides for probing fundamental scientific issues in electrokinetics. Over the past few years, several surprising theoretical predictions have been observed in experiments: (i) induced-charge electrophoresis of half-metallic Janus particles, perpendicular to a uniform AC field; (ii) microfluidic

A consideration of ethnographic data, computational model results, and archaeological data suggest that changes in family-level economics coincident with subsistence intensification contributed to the emergence of social complexity among prehistoric hunter–gatherers in eastern North America by creating the conditions for a “rich get richer” scenario. Ethnographic data are used to construct a general computational model representing key person- and family-level behaviors, constraints, and decisions affecting the size and composition of hunter–gatherer families. Results from model experiments suggest that lowering the age at which children make a significant contribution to subsistence (e.g., through the broadening of the diet to include the kinds of mass-harvested, “low quality” foods that were increasingly exploited during the Archaic and Woodland periods) relaxes constraints on family size and makes large, polygynous families economically viable. Positive feedbacks between the productive and reproductive potentials of larger families produce right-tailed distributions of family size and “wealth” when the productive age of children is low and polygyny is incentivized. Size data from over 800 prehistoric residential structures suggest right-tailed distributions of family size were present during the Late Archaic through Middle Woodland periods. These distributions would have provided variability in family-based status that permitted the emergence of hereditary social distinctions.

People constantly talk about past experiences. Burgeoning psychologi- cal research has examined the role of communication in remembering by placing rememberers in conversational settings. In reviewing this work, we first discuss the benefits of collaborative remembering (trans- active memory and collaborative facilitation) and its costs (collaborative inhibition, information sampling biases, and audience tuning). We next examine how conversational remembering affects subsequent memory. Here, we address influences on listeners’ memory through social conta- gion, resistance to such influences, and then retrieval/reexposure effects on either speaker or listener, with a focus on retrieval-induced forget- ting. Extending the perspective beyond single interactions, we consider work that has explored how the above effects can spread across networks of several individuals. We also explore how a speaker’s motive to form a shared reality with listeners can moderate conversational effects on memory. Finally, we discuss how these various conversational effects may promote the formation of collective memories.

We consider networks of coupled maps where the connections between units involve time delays. We show that, similar to the undelayed case, the synchronization of the network depends on the connection topology, characterized by the spectrum of the graph Laplacian. Consequently, scale-free and random networks are capable of synchronizing despite the delayed flow of information, whereas regular networks with nearest-neighbor

Although it is commonly believed that women are kinder and more cooperative than men, there is conflicting evidence for this assertion. Current theories of sex differences in social behavior suggest that it may be useful to examine in what situations men and women are likely to differ in cooperation. Here we derive predictions from both sociocultural and evolutionary perspectives on context-specific sex differences in cooperation and conduct a unique meta-analytic study of 272 effect sizes -- sampled across 50 years of research -- on social dilemmas to examine several potential moderators. The overall average effect size is not statistically different from zero (d = -.05), suggesting that men and women do not differ in their overall amounts of cooperation. However, the association between sex and cooperation is moderated by several key features of the social context: Male-male interactions are more cooperative than female-female interactions (d = .16) yet women cooperate more than men in mixed-sex interactions (d = -.22). In repeated interactions men are more cooperative than women. Women were more cooperative than men in larger groups and in more recent studies, but these differences dissappeared after statistically controlling for several study characteristics. We discuss these results in the context of both sociocultural and evolutionary theories of sex differences, stress the need for an integrated biosocial approach, and outline directions for future research.

We present a scalable approach to dynamically allocating a swarm of homogeneous robots to multiple tasks, which are to be performed in parallel, following a desired distribution. We employ a decentralized strategy that requires no communication among robots. It is based on the development of a continuous abstraction of the swarm obtained by modeling population fractions and defining the task allocation problem as the selection of rates of robot ingress and egress to and from each task. These rates are used to determine probabilities that define stochastic control policies for individual robots, which, in turn, produce the desired collective behavior. We address the problem of computing rates to achieve fast redistribution of the swarm subject to constraint(s) on switching between tasks at equilibrium. We present several formulations of this optimization problem that vary in the precedence constraints between tasks and in their dependence on the initial robot distribution. We use each formulation to optimize the rates for a scenario with four tasks and compare the resulting control policies using a simulation in which 250 robots redistribute themselves among four buildings to survey the perimeters.