One objective of active matter science is to unveil principles by which chaotic microscale dynamics could be transformed into useful work. A nematic liquid crystal environment offers a number of possibilities, one of which is a directional motion of an active droplet filled with an aqueous dispersion of swimming bacteria. In this work, using the responsiveness of the nematic to the electric field and light, we demonstrate how to control the direction and speed of active droplets. The dielectric response of nematic to the electric field causes two effects: 1) reorientation of the overall director, and 2) changing the symmetry of the director configuration around the droplet. The first effect redirects the propulsion direction while the second one changes the speed. A laser beam pointed to the vicinity of the droplet can trigger the desired director symmetry around the droplet, by switching between dipolar and quadrupolar configurations, thus affecting the motility and polarity of propulsion. The dynamic tuning of the direction and speed of active droplets represents a step forward in the development of controllable microswimmers.
more »
« less
Directional self-locomotion of active droplets enabled by nematic environment
Active matter composed of self-propelled interacting units holds a major promise for the extraction of useful work from its
seemingly chaotic dynamics. Streamlining active matter is especially important at the microscale, where the viscous forces prevail
over inertia and transport requires a non-reciprocal motion. Here we report that microscopic active droplets representing
aqueous dispersions of swimming bacteria Bacillus subtilis become unidirectionally motile when placed in an inactive nematic
liquid-crystal medium. Random motion of bacteria inside the droplet is rectified into a directional self-locomotion of the droplet
by the polar director structure that the droplet creates in the surrounding nematic through anisotropic molecular interactions
at its surface. Droplets without active swimmers show no net displacement. The trajectory of the active droplet can be predesigned
by patterning the molecular orientation of the nematic. The effect demonstrates that broken spatial symmetry of the
medium can be the reason for and the means to control directional microscale transport.
more »
« less
- NSF-PAR ID:
- 10201953
- Date Published:
- Journal Name:
- Nature Physics
- ISSN:
- 1745-2473
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Self‐propulsion of highly wetting liquids is important in heat exchanger, air conditioning, and refrigeration systems. However, it is challenging to achieve such a spontaneous motion as these liquids tend to wet all the surfaces due to their ultralow surface tensions. Despite that extensive asymmetric surface structures and gradient chemical coatings are developed for directional droplet transport, they will be flooded and covered by these liquids. Here, this challenge is addressed by creating a gradient quasi‐liquid surface to achieve the self‐propulsion of droplets with surface tensions down to 10.0 mN m−1. Such a surface engineered by tethering flexible polymers with gradient grafting density shows ultralow contact angle hysteresis (<1o) to highly wetting liquids. Thus, the surface can simultaneously provide sufficient driving forces through the gradient wettability and negligible retention forces through the slippery boundary lubrication for spontaneous droplet movement. Moreover, continual self‐propulsion of tiny droplets is achieved by spraying highly wetting liquids in simulated condensation conditions and demonstrates that adding temperature gradient can further accelerate the self‐propulsion. The study provides a new paradigm to promote passive removal of highly wetting droplets, leading to potential impacts in enhancing condensation heat transfer regardless of surface orientations.
-
There is a recent surge of interest in the behavior of active particles that can at the same time align their direction of movement and synchronize their oscillations, known as swarmalators . While theoretical and numerical models of such systems are now abundant, no real-life examples have been shown to date. We present an experimental investigation of the collective motion of the nematode Turbatrix aceti that self-propel by body undulation. We discover that these nematodes can synchronize their body oscillations, forming striking traveling metachronal waves, which produces strong fluid flows. We uncover that the location and strength of this collective state can be controlled through the shape of the confining structure; in our case the contact angle of a droplet. This opens a way for producing controlled work such as on-demand flows or displacement of objects. We illustrate this by showing that the force generated by this state is sufficient to change the physics of evaporation of fluid droplets, by counteracting the surface-tension force, which allow us to estimate its strength. The relatively large size and ease of culture make Turbatrix aceti a promising model organism for experimental investigation of swarming and oscillating active matter capable of producing controllable work.more » « less
-
more » « less
Abstract Advanced synthetic materials are needed to produce nano‐ and mesoscale structures that function autonomously, catalyze reactions, and convert chemical energy into motion. This paper describes supracolloidal fiber‐like structures that are composed of self‐adhering, or “sticky,” oil‐in‐water emulsion droplets. Polymer zwitterion surfactants serve as the key interfacial components of these materials, enabling multiple functions simultaneously, including acting as droplet‐stabilizing surfactants, interdroplet adhesives, and building blocks of the fibers. This fiber motion, a surprising additional feature of these supracolloidal structures, is observed at the air–water interface and hinged on the chemistry of the polymer surfactant. The origin of this motion is hypothesized to involve transport of polymer from the oil–water interface to the air–water interface, which generates a Marangoni (interfacial) stress. Harnessing this fiber motion with functional polymer surfactants, and selection of the oil phase, produced worm‐like objects capable of rotation, oscillation, and/or response to external fields. Overall, these supracolloidal fibers fill a design gap between self‐propelled nano/microscale particles and macroscale motors, and have the potential to serve as new components of soft, responsive materials structures.
-
The isotropic to ferroelectric nematic liquid transition was theoretically studied over one hundred years ago, but its experimental studies are rare. Here we present experimental results and theoretical considerations of novel electromechanical effects of ferroelectric nematic liquid crystal droplets coexisting with the isotropic melt. We find that the droplets have flat pancake-like shapes that are thinner than the sample thickness as long as there is room to increase the lateral droplet size. In the center of the droplets a wing-shaped defect with low birefringence is present that moves perpendicular to a weak in-plane electric field, and then extends and splits in two at higher fields. Parallel to the defect motion and extension, the entire droplet drifts along the electric field with a speed that is independent of the size of the droplet and is proportional to the amplitude of the electric field. After the field is increased above 1 mV μm −1 the entire droplet gets deformed and oscillates with the field. These observations led us to determine the polarization field and revealed the presence of a pair of positive and negative bound electric charges due to divergences of polarization around the defect volume.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41567-020-01055-5
