Collaboration and competition between active sheets for self-propelled particles

In situ enzymatic control of colloidal phoresis and catalysis through hydrolysis of ATP

The ability to sense chemical gradients and respond with directional motility and chemical activity is a defining feature of complex living systems. There is a strong interest among scientists to design synthetic systems that emulate these properties. Here, we realize and control such behaviors in a synthetic system by tailoring multivalent interactions of adenosine nucleotides with catalytic microbeads. We first show that multivalent interactions of the bead with gradients of adenosine mono-, di- and trinucleotides (AM/D/TP) control both the phoretic motion and a proton-transfer catalytic reaction, and find that both effects are diminished greatly with increasing valence of phosphates. We exploit this behavior by using enzymatic hydrolysis of ATP to AMP, which downregulates multivalent interactivity in situ. This produces a sudden increase in transport of the catalytic microbeads (a phoretic jump), which is accompanied by increased catalytic activity. Finally, we show how this enzymatic activity can be systematically tuned, leading to simultaneous in situ spatial and temporal control of the location of the microbeads, as well as the products of the reaction that they catalyze. These findings open up new avenues for utilizing multivalent interaction-mediated programming of complex chemo-mechanical behaviors into active systems.

Engineering confined fluids to autonomously assemble hierarchical 3D structures

The inherent coupling of chemical and mechanical behavior in fluid-filled microchambers enables the fluid to autonomously perform work, which in turn can direct the self-organization of objects immersed in the solution. Using theory and simulations, we show that the combination of diffusioosmotic and buoyancy mechanisms produce independently controlled, respective fluid flows: one generated by confining surfaces and the other in the bulk of the solution. With both flows present, the fluid can autonomously join 2D, disconnected pieces to a chemically active, "sticky" base and then fold the resulting layer into regular 3D shapes (e.g. pyramids, tetrahedrons, and cubes). Here, the fluid itself performs the work of construction and thus, this process does not require extensive external machinery. If several sticky bases are localized on the bottom surface, the process can be parallelized, with the fluid simultaneously forming multiple structures of the same or different geometries. Hence, this approach can facilitate the relatively low-cost, mass production of 3D micron to millimeter-sized structures. Formed in an aqueous solution, the assembled structures could be compatible with biological environments, and thus, potentially useful in medical and biochemical applications.

Nonequilibrium thermodynamics of Janus particle self-assembly

We compute the energetic cost of formation of Janus particle structures. Using an approach that couples particle dynamics to the evolution of fuel concentration in the medium, which we consider to be initially inhomogeneous, we show the different types of emerging structures. The energy dissipated in the formation of such structures is obtained from the entropy production rate, which is a non-monotonic function of the fraction of assembled particles and, thus, different in each self-assembly regime. An analysis of the free energy of these particles allows us to establish a thermodynamic criterion of structure formation based on the behavior of chemical potential as a function of the fraction of assembled particles.

Chemical pumps and flexible sheets spontaneously form self-regulating oscillators in solution

The synchronization of self-oscillating systems is vital to various biological functions, from the coordinated contraction of heart muscle to the self-organization of slime molds. Through modeling, we design bioinspired materials systems that spontaneously form shape-changing self-oscillators, which communicate to synchronize both their temporal and spatial behavior. Here, catalytic reactions at the bottom of a fluid-filled chamber and on mobile, flexible sheets generate the energy to "pump" the surrounding fluid, which also transports the immersed sheets. The sheets exert a force on the fluid that modifies the flow, which in turn affects the shape and movement of the flexible sheets. This feedback enables a single coated (active) and even an uncoated (passive) sheet to undergo self-oscillation, displaying different oscillatory modes with increases in the catalytic reaction rate. Two sheets (active or passive) introduce excluded volume, steric interactions. This distinctive combination of the hydrodynamic, fluid-structure, and steric interactions causes the sheets to form coupled oscillators, whose motion is synchronized in time and space. We develop a heuristic model that rationalizes this behavior. These coupled self-oscillators exhibit rich and tunable phase dynamics, which depends on the sheets' initial placement, coverage by catalyst and relative size. Moreover, through variations in the reactant concentration, the system can switch between the different oscillatory modes. This breadth of dynamic behavior expands the functionality of the coupled oscillators, enabling soft robots to display a variety of self-sustained, self-regulating moves.

Controlling the Spatiotemporal Transport of Particles in Fluid-Filled Microchambers

The development of microscale devices that autonomously perform multistep processes is vital to advancing the use of microfluidics in industrial applications. Such advances can potentially be achieved through the use of "chemical pumps" that transduce the energy from inherent catalytic reactions into fluid flow within microchambers, without the need for extraneous external equipment. Using computational modeling, we focus on arrangements of multiple chemical pumps that are formed by anchoring patches of different enzymes onto the floor of a fluid-filled chamber. With the addition of the appropriate reactants, only one of the enzymatic patches is activated and thereby generates fluid flow centered about that patch. These flows drive the self-assembly of microparticles in the solution and localize the particles onto the activated patches. By varying the spatial arrangement of the enzymatic patches, and the sequence in which the appropriate reactants are added to the solution, we realize spatiotemporal control over the fluid flow and the sequential transport of microparticles from one patch to another. The order in which the particles visit the different patches can be altered by varying the sequence in which the reactants are added to the solution. By harnessing catalytic cascade reactions, where the product of one reaction is the reactant for the next, we achieve directed transport between the patches with the addition of just one reactant, which initiates the catalytic cascade. Through these studies, we show how the trajectory of the particles' motion among different "stations" can be readily regulated through intrinsic catalytic reactions and thus, provide guidelines for creating fluidic devices that perform multistep reactions in an autonomous, self-sustained manner.