The development of internal microstructure in electrospun fibers has been investigated both experimentally and theoretically. Various morphologies such as tubes, beads, and porous structures have been observed experimentally during electrospinning from amorphous polymer solutions such as poly(methyl methacrylate)/methylene chloride and poly(styrene)/tetrahydrofuran. The dynamics of electrospinning is modeled based on multiple virtual strands of beads connected by Maxwell's elements in a cylindrical coordinate system. Concurrently, spatiotemporal growth of the porous structure is calculated in the framework of Cahn-Hilliard time-evolution equation under the quasi steady state assumption coupled with the solvent evaporation rate equation. The coarse-grain simulation reveals the real-time formation of pores along the spinline of the electrospun fiber as the concentration traverses across the phase diagram of the amorphous polymer solution.

We undertake the first computational study to determine the effect of light on polymer gels undergoing the Belousov-Zhabotinsky (BZ) reaction. The BZ gels are unique materials because they can undergo rhythmic mechanical oscillations in the absence of external stimuli. The BZ reaction, however, is photosensitive. Via simulations, we demonstrate that the interplay between the chemoresponsive gels and the photosensitive reaction can cause millimeter sized BZ gels to exhibit autonomous, directed motion or reorientation away from 4 the light. In effect, we show that these synthetic BZ "worms" display a fundamental biomimetic behavior: movement away from an adverse environmental condition, which in the context of the BZ reaction is the presence of light.

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 review advances in a new area of interdisciplinary research that concerns phenomena arising from inherent coupling between non-linear chemical dynamics and mechanics. This coupling provides a route for chemical-to-mechanical energy transduction, which enables materials to exhibit self-sustained oscillations and/or waves in both concentration and deformation fields. We focus on synthetic polymer gels, where the chemo-mechanical behavior can be engineered into the material. We provide a brief review of experimental observations on several types of chemo-mechanical oscillations in gels. Then, we discuss methods used to theoretically and computationally model self-oscillating polymer gels. The rest of the paper is devoted to describing results of theoretical and computational modeling of gels that undergo the oscillatory Belousov-Zhabotinsky (BZ) reaction. We discuss a remarkable form of mechano-chemical transduction in these materials, where the application of an applied force or mechanical contact can drive the system to switch between different dynamical behavior, or alter the mechanical properties of the material. Finally, we discuss ways in which photosensitive BZ gels could be used to fabricate biomimetic self-propelled objects. In particular, we describe how non-uniform illumination can be used to direct the movement of BZ gel 'worms' along complex paths, guiding them to bend, reorient and turn.

Temporal evolution of the fiber morphology during dry spinning has been investigated in the framework of Cahn-Hilliard equation ͓J. Chem. Phys. 28, 258 ͑1958͔͒ pertaining to the concentration order parameter or volume fraction given by the Flory-Huggins free energy of mixing

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Summary: To mimic the emergence of gradient morphology in polymer nanofibers, a new theoretical approach has been developed in the context of Cahn‐Hilliard time evolution equation, alternatively known as time‐dependent Ginzburg‐Landau equation (Model B) involving concentration order parameter. The effects of solvent evaporation on the morphology evolution of the nanofibers have been demonstrated. The numerical simulation showed that the formation of skin layers is governed by the competition between solvent evaporation rate and mutual diffusion rate. That is to say the skin layers are formed in the nanotube whenever the rate of evaporation exceeds a critical value; otherwise, a solid fiber is formed. In hollow nanofibers, the layer can grow to a substantial fraction of the fiber diameter, allowing it to remain intact, albeit often in a collapsed form.

Using computational modeling, we show that self-oscillating Belousov-Zhabotinsky (BZ) gels can both emit and sense a chemical signal and thus drive neighboring gel pieces to spontaneously self-aggregate, so that the system exhibits autochemotaxis. To the best of our knowledge, this is the closest system to the ultimate selfrecombining material, which can be divided into separated parts and the parts move autonomously to assemble into a structure resembling the original, uncut sample. We also show that the gels' coordinated motion can be controlled by light, allowing us to achieve selective self-aggregation and control over the shape of the gel aggregates. By exposing the BZ gels to specific patterns of light and dark, we design a BZ gel "train" that leads the movement of its "cargo." Our findings pave the way for creating reconfigurable materials from self-propelled elements, which autonomously communicate with neighboring units and thereby actively participate in constructing the final structure.self-oscillating gels | autochemotactic gels | autochemotactic self-organization S pecies ranging from single-cell organisms to social insects can undergo autochemotaxis, where the entities move toward a chemo-attractant that they themselves emit. This mode of signaling allows the organisms to form large-scale structures, with amoebas (1) and Escherichia coli (2) self-organizing into extensive multicellular clusters and termites constructing macroscopic mounds (3). Notably there are few equivalents of such autochemotactic-driven assembly in the synthetic world. Although researchers have devised a range of nano-and microscopic selfpropelled particles (4), hardly any exhibit autochemotaxis (5) that leads to the formation of extended structures (6). Although recent theoretical models provide insight into the autochemotaxis of a self-propelled walker (7) and active Brownian particles (8), these studies provide few guidelines for synthesizing specific materials that display autochemotactic self-organization. The latter materials would open new routes for dynamic, reconfigurable self-assembly, where self-propelled elements communicate with neighboring units and thereby actively participate in constructing the final structure. Herein, we use computational modeling to show that millimeter-sized polymer gels can display such self-sustained, autochemotatic behavior. In particular, we demonstrate that gels undergoing the self-oscillating Belousov-Zhabotinsky (BZ) reaction (9) not only respond to a chemical signal from the surrounding solution, but also emit this signal and thus, multiple neighboring gel pieces can spontaneously self-aggregate into macroscopic objects. These findings indicate that BZ gels can undergo a form of "selfrecombining": if a BZ gel is cut into distinct pieces and the pieces are moved relatively far apart, then their autochemotactic behavior drives the parts to move autonomously and recombine into a structure resembling the original, uncut sample (Fig. 1). We also show that the gels' coordinated motion can ...