Microrollers Flow Uphill as Granular Media

Pour sand into a container and only the grains near the top surface move. The collective motion associated with the translational and rotational energy of the grains in a thin flowing layer is quickly dissipated as friction through multibody interactions. Alternatively, consider what will happen to a bed of particles if one applies a torque to each individual particle. In this paper, we demonstrate an experimental system where torque is applied at the constituent level through a rotating magnetic field in a dense bed of microrollers. The net result is the grains roll uphill, forming a heap with a negative angle of repose. Two different regimes have been identified related to the degree of mobility or fluidization of the particles in the bulk. Velocimetry of the near surface flowing layer reveals the collective motion of these responsive particles scales in a similar way to flowing bulk granular flows. A simple granular model that includes cohesion accurately predicts the apparent negative coefficient of friction. In contrast to the response of active or responsive particles that mimic thermodynamic principles, this system results in macroscopic collective behavior that has the kinematics of a purely dissipative granular system

Pattern engineering of living bacterial colonies using meniscus-driven fluidic channels

Creating adaptive, sustainable, and dynamic biomaterials is a forthcoming mission of synthetic biology. Engineering spatially organized bacterial communities has a potential to develop such bio-metamaterials. However, generating living patterns with precision, robustness, and a low technical barrier remains as a challenge. Here we present an easily implementable technique for patterning live bacterial populations using a controlled meniscus-driven fluidics system, named as MeniFluidics. We demonstrate multiscale patterning of biofilm colonies and swarms with submillimeter resolution. Utilizing the faster bacterial spreading in liquid channels, MeniFluidics allows controlled bacterial colonies both in space and time to organize fluorescently labeled Bacillus subtilis strains into a converged pattern and to form dynamic vortex patterns in confined bacterial swarms. The robustness, accuracy, and low technical barrier of MeniFluidics offer a tool for advancing and inventing new living materials that can be combined with genetically engineered systems, and adding to fundamental research into ecological, evolutional, and physical interactions between microbes

Transition to collective motion of spermatozoa in different confinements and at variable temperatures

Sperm motility and its collective motion is a subject still poorly investigated. We aim at studying different environmental conditions that can and do affect the motility of sperm cells, focusing into transitions to collective motion. With increasing concentration of swimmers, sperm cells in a suspension can switch from random motion to organised collective turbulence, which we call "spermulence". This phase transition is strongly influenced by the boundaries of the system, which influence conditions for the transition to spermulence. Complexity of the boundary may lead to oscillatory modes where the flow in suspension reverses periodically. Moreover, the confinement can be used by the swimming sperm cells as an efficient strategy to progress towards a point and this particular phenomenon can be important when designing methods for artificial insemination. In addition to the confinement, other environmental conditions, such as temperature and fluid viscosity, would influence the motion of sperm cells. We demonstrate that cells change the radius of curvature of their trajectories when swimming in a hotter environment. This change in trajectory will result in formation of ring-like structures in a two dimensional system, thatwill turn into a oscillatory motion,with waves propagating throughout the entire system. Self-organisations on such a wide scale and with such consistency have not been yet seen in the spermatozoa investigations. While working with sperm cells, a new method for swarming bacterial experiments has been invented, allowing for bacteria to live on an agar plate and move in the space for long times compared to their usual live span in devices. This techniques allow bacteria to swarmfor hours on agar surface previously modified with the desired structure

Microrollers flow uphill as granular media

Abstract Pour sand into a container and only the grains near the top surface move. The collective motion associated with the translational and rotational energy of the grains in a thin flowing layer is quickly dissipated as friction through multibody interactions. Alternatively, consider what will happen to a bed of particles if one applies a torque to each individual particle. In this paper, we demonstrate an experimental system where torque is applied at the constituent level through a rotating magnetic field in a dense bed of microrollers. The net result is the grains roll uphill, forming a heap with a negative angle of repose. Two different regimes have been identified related to the degree of mobility or fluidisation of the particles in the bulk. Velocimetry of the near surface flowing layer reveals the collective motion of these responsive particles scales in a similar way to flowing bulk granular flows. A simple granular model that includes cohesion accurately predicts the apparent negative coefficient of friction. In contrast to the response of active or responsive particles that mimic thermodynamic principles, this system results in macroscopic collective behavior that has the kinematics of a purely dissipative granular system