Chemical Patterning

Figure C2.4.12. Principle of the microcontact printing process. Chemically patterned organic films witli differently fimctionalized regions can be prepared by a combination of pCP and subsequent immersion.

Analogous considerations apply to spatially distributed reacting media where diffusion is tire only mechanism for mixing chemical species. Under equilibrium conditions any inhomogeneity in tire system will be removed by diffusion and tire system will relax to a state where chemical concentrations are unifonn tliroughout tire medium. However, under non-equilibrium conditions chemical patterns can fonn. These patterns may be regular, stationary variations of high and low chemical concentrations in space or may take tire fonn of time-dependent stmctures where chemical concentrations vary in botli space and time witli complex or chaotic fonns. 

In tills chapter we shall examine how such temporal and spatial stmctures arise in far-from-equilibrium chemical systems. We first examine spatially unifonn systems and develop tlie tlieoretical tools needed to analyse tlie behaviour of systems driven far from chemical equilibrium. We focus especially on tlie nature of chemical chaos, its characterization and the mechanisms for its onset. We tlien turn to spatially distributed systems and describe how regular and chaotic chemical patterns can fonn as a result of tlie interjilay between reaction and diffusion. 

We shall describe some of tire common types of chemical patterns observed in such experiments and comment on tire mechanisms for tlieir appearance. In keeping witli tire tlieme of tliis chapter we focus on states of spatio-temporal chaos or on regular chemical patterns tliat lead to such turbulent states. We shall touch only upon tire main aspects of tliis topic since tliere is a large variety of chemical patterns and many mechanisms for tlieir onset [2,3, 5,31]. 

If tlie diffusion coefficients of tlie chemical species are sufficiently different, new types of chemical instability arise which can lead to tlie fonnation of chemical patterns and ultimately to spatio-temporal chaotic behaviour. 

Consequently, when D /Dj exceeds the critical value, close to the bifurcation one expects to see the appearance of chemical patterns with characteristic lengtli i= In / k. Beyond the bifurcation point a band of wave numbers is unstable and the nature of the pattern selected (spots, stripes, etc.) depends on the nonlinearity and requires a more detailed analysis. Chemical Turing patterns were observed in the chlorite-iodide-malonic acid (CIMA) system in a gel reactor [M, 59 and 60]. Figure C3.6.12(a) shows an experimental CIMA Turing spot pattern [59]. 

Several important topics have been omitted in this survey. We have described only a few of the routes by which chaos can arise in chemical systems and have made no attempt to describe in detail the features of the different kinds of chemical strange attractor seen in experiments. A wide variety of chemical patterns have been observed and while the many aspects of the mechanisms for their appearance are understood, some features like nonlinear... 

Heterogeneity extends beyond the geometric, and chemical patterning can also be critical. Chemically patterned release surfaces can result in various levels of... 

An attempt has been made to bring out any variations there may be in processing methodology that would produce a different chemical pattern in the final beverage. 

Clayton, R. 1980. Photosynthesis Physical Mechanisms and Chemical Patterns. Cambridge, U.K. Cambridge University Press. 

Clayton, R.K. "Photosynthesis Physical Mechanisms and Chemical Patterns" Cambridge University Press Cambridge, England, 1980. 

Figure 11.4. Fluidic flow-directed assembly of NWs. (a,b) Schematic (a) and SEM image (b) of regular NW arrays obtained by flowing NW solution over a chemically patterned surface. (c,d) Parallel and crossed NW device arrays obtained with fluidic flow assembly. [Adapted from Refs. 44 and 49.]...

Isotopic and chemical patterns are given for an ambient sample (downtown Portland) and four sources of carbonaceous particles. The source C contributions (Ct) are expressed as percent of total aerosol mass, and the subscripts refer to Vegetation (slash burning), Road dust, Oil (residual and distillate), and Auto exhaust. Units for the ordinate are also percent of total aerosol mass, except for, 4C which is expressed as dpm/g-aerosol. Abscissa points for each pattern are ordered as l4C, C, Si, Ni, and Pb. 

In addition to the aforementioned methods for controlling substrate-polymer interactions uniformly across the entire surface, the use of top-down lithographic techniques to chemically pattern substrates provides spatial control over these substrate-polymer interactions and therefore provides even... 

The formation of bottom-up block copolymer patterns within or on top-down substrate patterns is the basis for so-called templated self-assembly processes, in which long-range order and orientation of microdomain patterns can be imposed by a template or guide . These top-down templates can take a variety of forms including periodic thickness profiles and chemically patterned surfaces. 

By modifying the surface/interface energies in a periodic manner, chemically patterned surfaces have also been widely used to control the orientation of nanostructures over large areas (Fig. 6). Theoretical [138-145] and experimental [73-76,88,89,146] results have indicated that with the appropriate surface grating and boundary conditions, lateral control over nanostructures propagates microns away from the surface (deep into the film), thus providing true 3D control of the self-assembly process. Russell and coworkers [73,74]... 

Fig. 6 Schematic representation of controlling the block copolymer thin film orientations by using top-down lithography-defined chemically patterned heterogeneous sin-face...

London. On the second day, the late afternoon peak exceeded 0.1 ppm. It is apparent that some conversion of nitric oxide to nitrogen dioxide was responsible for the ozone buildups, but it is not as clear a chemical pattern for London as it is for cities in the western United States. The paper did not mention the method of ozone measurement however, qualitative descriptions of the weather patterns suggest that the days of high ozone were characterized by light winds and considerable sunlight. 

O. Strouf, Chemical Pattern Recognition, Research Smdies Press, Letchworth, 1986. 

An alternative to the delivery techniques described previously involves using surface chemistry alone to assist in site-selective attachment of NAs at predetermined locations. This approach relies on the generation of a surface chemical pattern possessing adsorbing and nonadsorbing regions, in such... 

Fig. 23 Site-selective attachment of NAs onto chemically patterned surfaces...

Various fabrication schemes have emerged using this approach. A common one is the site-selective photochemical etching of an organosilane-modified surface by irradiation through a mask [86]. This results in a chemically patterned surface with active sites, where modified ONDs can be attached, surrounded by bare (irradiated) regions. 

Alternatively, chemical patterns can be generated by photohthography of a photoresist layer that has been spin-coated over a silane film. Upon irradiation through a mask and subsequent development of the photoresist pattern, the imderlying modified layer is revealed and is able to react with the ONDs. By sequentially irradiating the surface, it is possible to immobihze different ONDs. 

A new methodology, which combines traditional (top-down) lithography and self-assembly (bottom-up) approaches, has been developed for fabrication of complex nanoscale colloidal stmctures on a surface over a large domain. The principle of this method is to create a chemically patterned surface by well-established soft lithography, followed by self-assembling nanoparticles selectively deposited... 

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