Prior to the launching of Project VULCAN and following results that inspired and motivated VULCAN, we discovered that sticky particles immersed in suspensions of swimming E. coli bacteria assemble into unconventional gels. The structures of the gels is controlled by an unexpected rotation of the passive clusters as they aggregate. We identified that swimming bacteria generate this clockwise torque, making the fluid around them “chiral” and capable of inducing persistent rotations in colloidal clusters - effectively a chiral bacterial baths of swimming E. coli.
Swimming bacteria move through fluid by actuation of their moving body parts. It results that the forces they exert on their surroundings are internal forces: in other words, swimming bacteria are force-free and exert zero total external force. In effect, they can be modeled to lowest order as force dipoles and are classified hydrodynamically as pushers or pullers depending on the direction of the hydrodynamic dipole and to state it briefly the force dipole description of active matter particles has become the gold standard of theoretical modeling. However, this view only accounts for the translational motion of the swimming body. In fact, bacteria are also torque-free. For example, the body of flagellated E. coli bacteria rotates one way, their flagella the other way: they are torque dipoles. To date, this effect has only been discussed as the origin of why E. coli bacteria swim in circles near interfaces.
Under Objective 1, we showed that this torque dipole has important implications, allowing, for example, E. coli bacteria to rotate objects much larger than themselves. In contrast with the seminal works that relied on a ratcheting of the collisions with the bacteria to power gears, we show that the torque dipole transmits work in a contactless fashion in situations agnostic to the shape of the object, thus providing remarkable and desirable robustness at the microscale. The impact of torque dipoles of bacteria on their surroundings has been, to date, totally overlooked. We demonstrate that confined hydrodynamic interactions are a potent tool to bridge scales, leveraging the chirality of the bacterial flagellar nanomotors to power objects many orders of magnitude larger. Our results show most impact when bacteria are confined, an ubiquitous situation in ecological (e.g. bacteria in porous materials) or biological settings, with dense biofilms where bacteria stack promiscuously. This mechanism has potential to devise chiral fluids and highlight the potent role of bacterial torque dipoles in confined situations, relevant in microbial environments such as porous materials or biofilms. Building on this, we are developing optogenetic bacterial baths, where light controls bacterial swimming speed. This allows us to locally tune bacterial activity and spatially program the formation of colloidal gels.
In the last decade, the field of active matter has had a major impact across disciplines, deriving general descriptions of active fluids that explained a multiple of living and synthetic phenomena across scales, from animal flocks to runner crowds, subcellular self-organization and non-equilibrium phase transitions. However, active fluids are fluids: they flow easily (even spontaneously!) and cannot sustain forces – which makes them often useless from a practical engineering standpoint. Yet, the addition of activity to matter holds the key to pressing challenges of engineering: from hierarchical self-organization to smart materials. This has motivated over the last couple years the exploration of active solids, solids made of active constituents elastically coupled with each other, to achieve dynamical or shape-shifting materials.
Under Objective 2, we created active beams from dozens of active colloids and unveil their surprising dynamics. Active beams are active solids, whereby each building block continuously consumes energy, and an unconventional twist to the classical elastic beam of mechanical textbooks. We unveil a variety of complex behaviors such as self-oscillations or persistent rotations, that are, remarkably, controlled by elemental changes in their boundary conditions and mapped onto phase transitions. We quantitatively reproduce our experimental results with a first-principle minimal model of an elastic beam driven by active particles, coupled with hydrodynamic feedback. Beyond, the experimental tour de force, thus work highlights an unconventional roadmap to engineering dynamical behaviors and oscillations and provides insights into the novel class of materials that are active solids and more broadly into biological systems. Finally, we believe that this work will inspire novel designs in micro and nano-machines, notably connecting to the emerging fields of colloidal and swarm robotics
As a additional result to the original plan, we showed that cut patterns in liquid crystal elastomers (LCEs) program mechanical behavior and soft robotics functions by programming strain fields that couple to molecular properties of LCEs, turning internal stress into function (=matter powered from within) and making LCE kirigami more that the sum of their parts.
Together, these results demonstrate how we can design and understand matter powered from within, opening new possibilities for active and self-organizing materials.
