(Color) (a) Chameleons, such as this Chamaeleo chameleon, are green when calm (left) but turn dark brown when threatened (right). (b) Squids of the same species display different colors depending on their “mood” and environment. (c) Close up of the skin of the squid Lolliguncula brevis; note the collection of light-reflecting iridophores. (d) Schematic illustration of RD amplification of a nanometer scale “signal” into a macroscopic, visual “readout.” [Image credits: (a, left) unknown photographer; (a, right) Unknown photographer, December 2005; (b-c) courtesy of cephbase.com.]
(Color) (a) Schematic illustration of the basic experimental arrangement for WETS. Here, an agarose stamp micropatterned in bas relief is soaked in a solution of an inorganic salt (e.g., ) and applied onto a dry gelatin film containing another salt [e.g., ]. Typical dimensions are , , , , and . (b) Qualitative description of the reaction-diffusion processes inside of a patterned gel. Aqueous solution of salt diffuses from the stamp into the gelatin layer, where it precipitates to give a colored (here, Prussian blue). Unprecipitated experiences a concentration gradient (red line) that causes it to diffuse towards the incoming . Because the diffusion coefficient of depends nonlinearly on the amount of precipitate produced at location , , the counterpropagating color fronts are sharp. When all is consumed, these fronts come to a halt very close (down to ) to one another, leaving a clear line in between. Mathematical details of this phenomenon are described in detail in Ref. 36 . (c) Optical micrograph of a square grid of clear lines developed by wet-stamping a square array of circles. Scale bar is . (d) WETS also allows for patterning of dynamic patterns. The image shows chemical waves traveling through an excitable BZ medium and originating from a pentagonal star. Note that the waves are emitted discontinuously—this effect is discussed in detail in Sec. VI and also in Fig. 7 .
RD “adaptive” patterning with micronetworks. (a) The scheme illustrates Wet Stamping of a network of connected features (here, triangular lattice) and defines pertinent dimensions. (b) Qualitative water-content, , and concentration profiles across an edge (left), and along it (right). The numbers correspond to the shaded planes in (a). (c) The diagrams illustrate the directions of flow of in agarose (thin arrows) and directions of the propagation of RD fronts in gelatin (thick arrows) for TC (left) and DL (right) solutions. Experimental images of the stamp/gel interface developing into a TC tiling (left, , , ) and a DL tiling (right, , , ).
Sensing substrate thickness with micronetworked stamps. (a) Definition of a tile-centered, TC, and a dual-lattice, DL, transformations (both dashed lines) of a stamped triangular network (solid lines). (b) Applying identical micronetworks onto thinner ( , left column) and thicker ( , right column) gels results in dramatic changes in the visual readout, with thin and thick gels giving TC and DL tiling transformations, respectively. The transition from TC to DL solutions occurs over differences in gel thickness as small as a few .
(Color) RD visualization of phase transitions. (a) Schematic illustration (left) of a low-symmetry network pattern, containing both three and four type nodes. Qualitative profiles (right) of the concentrations of water (red lines) and iron cations (blue lines) along a 3-4 edge, leading to symmetric (top) and asymmetric (bottom) RD solutions. (b) Percent of asymmetric solutions as a function of gelatin’s temperature. RD patterns propagate either only from the nodes (“asymmetric;” below ) or from both types of nodes (“symmetric;” above ), where is the distance between and nodes. (c) DSC scan of a gelatin/hexacyanoferrate solution showing a helix-to-coil transition at equal to that of the crossover between asymmetric and symmetric RD patterns on dry gel films.
Detection of self-assembled monolayers of alkane thiolates on gold. (a) When is delivered to a dry gelatin layer doped with and supported on a clean gold substrate, PP patterns form to give visible bands of (dark gray). If, however, the gold surface is covered with a SAM of 11-mercaptoundecanol (shown above the diagrams), the precipitation of proceeds continuously with no visible banding. (b) In the absence of the electron-insulating SAM, oxidation of to sets up an appropriate PP chemistry. This redox reaction is eliminated by the presence of the SAM, which prevents the transfer of electrons. (c) Scanning electron microscopy images of the phenomenon. Left: PP patterns form on a bare gold substrate. Right: Continuous and uniform precipitation occurs in the presence of an insulating SAM. Scale bars are .
Dynamic “metabolic” response of a chemical “starfish.” (a) Kinetic model accounting for perturbations due to formaldehyde and methanol (see Ref. 68 for details). Here, , , , , , products (i.e., , COOH, ), HOBr, (i.e., or ), , , , , and is a stoichiometric factor. (b) Qualitative illustration of activator- and inhibitor-controlled pattern formation. Here, food (formaldehyde) favors the production of inhibiting species, which flood the regions between the star’s arms, thereby inhibiting wave emission from those areas. At the tips, the inhibiting species are sufficiently diluted due to the curvature of the region, and waves are initiated selectively at those locations. The situation is reversed in the case of food (methanol) where the activator is diluted at the star’s tips while flooding the regions between the arms. (c) Experimental images obtained after wet-stamping formaldehyde and (d) methanol.
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