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Biological cells communicate through a complex chemical process where a signaling cell secretes molecules that are then detected by receptors on the target cell. This passage of information allows the cells to cooperate and thereby carry out a diverse range of functions. Herein, we use computational modeling and theory to design a synthetic system that effectively mimics this biological process. In particular, we show how one polymeric microcapsule "signals" to another and thereby initiates the motion of both. Microcapsules (such as those fabricated by the sequential deposition of oppositely charged chains 1 ) provide optimal features for performing this biomimetic behavior: they encompass a porous shell and a fluid-filled core, which can contain nanoparticles.2 As encapsulated nanoparticles diffuse through the porous shell and into the surrounding fluid, they act as signaling species and the capsule can be viewed as secreting the vital compounds. In our scenario, both the signaling and the target microcapsules sit on an initially homogeneous adhesive surface. Neither capsule alone can move along this surface; however, the nanoparticle-facilitated communication between the two promotes and sustains collective, directed motion.While researchers have focused on isolating factors that promote the self-directed motion of individual liquid droplets, [3][4][5][6][7][8][9][10][11][12][13][14][15] there have been few studies to examine how collective behavior between droplets 16 or capsules can lead to self-propelled movement. Our findings can provide insight into fundamental physical processes that control chemotaxis between biological cells. 17In addition, polymeric capsules are finding use as microreactors and the results yield guidelines for manipulating their interactions in microfluidic devices.Using a novel computational approach (see Methods), we simulate the behavior of the system shown in Figure 1; the first capsule (the signaling capsule) contains dispersed nanoparticles in its fluid-filled core, while the second fluid-filled capsule (the target capsule) is initially devoid of nanoparticles. Each capsule's two-dimensional, rigid shell is composed of two layers of lattice nodes, which make up the inner and outer surfaces. We assume that the nanoparticles can diffuse through this shell, 18 but the fluid content of the capsule remains constant. The released nanoparticles can chemisorb onto the substrate and the adsorbed nanoparticles modify the wetting properties of the surface. 19 In particular, the strength of the adhesive interaction between the capsules and surface decreases with the fractional surface coverage of nanoparticles. The adsorbing nanoparticles can thereby create an adhesion gradient along the surface, and if the gradient is sufficiently asymmetric, 3-16 a capsule could be driven by enthalpic forces to move from

Dr. Spudis earned his master's degree from Brown University and his Ph.D. from Arizona State University in Geology with a focus on the Moon. His career included work at the US Geological Survey,

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The term integrated solar combined-cycle (ISCC) has been used to define the combination of solar thermal energy into a natural gas combined-cycle (NGCC) power plant. Based on a detailed thermodynamic cycle model for a reference ISCC plant, the impact of solar addition is thoroughly evaluated for a wide range of input parameters such as solar thermal input and ambient temperature. It is shown that solar hybridization into an NGCC plant may give rise to a substantial benefit from a thermodynamic point of view. The work here also indicates that a significant solar contribution may be achieved in an ISCC plant, thus implying substantial fuel savings and environmental benefits.