No abstract

A single biological cilium can sense minute chemical variations and transmit this information to neighboring cilia to produce a global response to the local change. Herein, we undertake the first computational study of self-oscillating, artificial cilia and show that this system can ''communicate'' to undergo a biomimetic, collective response to small-scale chemical changes. The cilia are formed from chemo-responsive gels undergoing the oscillatory Belousov-Zhabotinsky (BZ) reaction. The activator for the reaction, u, is generated within these BZ cilia and diffuses between the neighboring gels. We find that the spatial arrangement of the BZ cilia affects the local distribution of u, which in turn affects the dynamic behavior of the system. Consequently, two closely spaced cilia bend away from each other and the chemo-mechanical traveling waves within the gels propagate top down. By increasing the inter-cilia spacing, we dramatically alter the behavior of the system and uncover a distinctive form of chemotaxis: the tethered gels bend towards higher concentrations of u and hence, towards each other. This chemotaxis is particularly pronounced in an array of five cilia, where we observe a ''bunching'' of the cilia towards the highest concentration in u, accompanied by the synchronization of the chemomechanical waves. We also show that the cilial oscillations can be controlled remotely and noninvasively by light. By selectively illuminating certain cilia, we could ''play'' the array like a keyboard, causing a rhythmic variation in the heights of the gels. These attributes could be exploited in a range of microfluidic applications, where the controllable communication among the BZ cilia and selfoscillating surface topology can be harnessed to transport microscopic objects within the devices.

We model the transport of a microscopic particle via a regular array of beating elastic cilia, whose tips experience an adhesive interaction with the particle's surface. At optimal adhesion strength, the average particle velocity is maximized. Using simulations spanning a range of cilia stiffness and cilia-particle adhesion strength, we explore the parameter space over which the particle can be "released", "propelled", or "trapped" by the cilia. We use a lower-order model to predict parameters for which the cilia are able to "propel" the particle. This is the first study that shows how both stiffness and adhesion strength are crucial for manipulation of particles by active cilia arrays. These results can facilitate the design of synthetic cilia that integrate adhesive and hydrodynamic interactions to selectively repel or trap particulates. Surfaces that are effective at repelling particulates are valuable for antifouling applications, while surfaces that can trap and, thus, remove particulates from the solution are useful for efficient filtration systems.

In this paper, we present our recent experimental results on buoyancy-induced convection in aluminum metal foams of different pore densities [corresponding to 5, 10, 20, and 40 pores per in. (PPI)] and porosities (0.89–0.96). The results show that compared to a heated surface, the heat transfer coefficients in these heat sinks are five to six times higher. However, when compared to commercially available heat sinks of similar dimensions, the enhancement is found to be marginal. The experimental results also show that for a given pore size, the heat transfer rate increases with porosity, suggesting the dominant role played by conduction in enhancing heat transfer. On the other hand, if the porosity is held constant, the heat transfer rate is found to be lower at higher pore densities. This can be attributed to the higher permeability with the larger pores, which allows higher entrainment of air through the porous medium. New empirical correlations are proposed for the estimation of Nusselt number in terms of Rayleigh and Darcy numbers. We also report our results on novel finned metal foam heat sinks in natural convection. Experiments were conducted on aluminum foams of 90% porosity with 5 and 20 PPI with one, two, and four aluminum fins inserted in the foam. All of these heat sinks were fabricated in-house. The results show that the finned metal foam heat sinks are superior in thermal performance compared to the normal metal foam and conventional finned heat sinks. The heat transfer increases with an increase in the number of fins. However, the relative enhancement is found to decrease with each additional fin. The indication is that there exists an optimum number of fins beyond which the enhancement in heat transfer, due to increased surface area, is offset by the retarding effect of overlapping thermal boundary layers. Similar to normal metal foams, the 5 PPI samples are found to give higher values of h compared to the 20 PPI samples due to higher permeability of the porous medium. Future work is planned to arrive at the optimal heat sink configuration for even larger enhancement in heat transfer.

Evolution is a characteristic feature of living systems, and many fundamental processes in life, including the cell cycle, take place in a periodic fashion. From a chemistry perspective, these repeating phenomena suggest the question of whether reactions in which concentrations oscillate could provide a basis and/or useful models for the behavior of organisms, and perhaps even their ability to evolve. In this Account, we examine several aspects of the behavior of the prototype oscillating chemical reaction, the Belousov-Zhabotinsky (BZ) system, carried out in microemulsions, arrays of micrometer-sized aqueous droplets suspended in oil, or hydrogels. Each of these environments contains elements of the compartmentalization that likely played a role in the development of the first living cells, and within them we observe behaviors not found in the BZ reaction in simple aqueous solution. Several of these phenomena resemble traits displayed by living organisms. For example, the nanodroplets in a BZ microemulsion "communicate" with each other through a phenomenon analogous to quorum sensing in bacteria to produce a remarkable variety of patterns and waves on length scales 10(5) times the size of a single droplet. A photosensitive version can "remember" an imposed image. Larger, micrometer-sized droplets exhibit similarly rich behavior and allow for the observation and control of individual droplets. These droplets offer promise for building arrays capable of computation by varying the strength and sign of the coupling between drops. Gels that incorporate a BZ catalyst and are immersed in a solution containing the BZ reactants change their shape and volume in oscillations that follow the variation in the redox state of the catalyst. Using this phenomenon, we can construct phototactic gel "worms" or segments of gel that attract one another. Whether such systems will provide more realistic caricatures of life, and whether they can serve as useful materials will largely depend on the successful integration of various properties, including communication, motion, and memory, which we observed in separate experiments. Theoretical approaches that couple reaction and diffusion processes to mechanical and other material properties are likely to play a key role in this integration, and we describe one such approach. The evolution of systems of coupled chemical oscillators presents another challenge to the development of these systems, but one that we expect to be solved.

Species ranging from single-cell organisms to social insects can undergo auto-chemotaxis, where the entities move towards a chemo-attractant that they themselves emit. Polymer gels undergoing the self-oscillating Belousov-Zhabotinsky (BZ) reaction exhibit autonomous, periodic pulsations, which produce chemical species collectively referred to as the activator. The diffusion of this activator into the surrounding solution affects the dynamic behavior of neighboring BZ gels and hence, the BZ gels not only emit, but also respond to self-generated chemical gradients. This review describes recent experimental and computational studies that reveal how this biomimetic behavior effectively allows neighboring BZ gels to undergo cooperative, self-propelled motion. These distinctive properties of the BZ gels provide a route for creating reconfigurable materials that autonomously communicate with neighboring units and thereby actively participate in constructing the desired structures.

Inspired by the ability of marine suspension feeders to selectively capture small particles by their hairlike cilia, we simulate the interaction between artificial cilia and microscopic particles of different sizes to determine if a purely synthetic system can display analogous size-selective behavior. Our computational approach specifically models the capture of particles suspended in the surrounding fluid by adhesive filaments, which are anchored by one end to a surface. Via this model, we show that this size selectivity can arise as a result of adhesive and hydrodynamic interactions in the system. The substantial reduction in the mobility of the large particles near surfaces leads to a failure in capturing large particles. Using a simple analytical model, we show that the balance of hydrodynamic and adhesive forces favors capture of particles below a critical size for a given cilia-particle interaction. Our findings provide guidelines for designing artificial cilia that can be used for sorting and transporting particles within microfluidic devices.

No abstract