The ability to design and assemble 3-dimensional structures from colloidal particles is limited by the absence of specific directional bonds. As a result, complex or low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain. Here we demonstrate a general method for creating the colloidal analogues of atoms with valence: colloidal particles with chemically functionalized patches that can form highly directional bonds.These "colloidal atoms" possess all the common symmetries-and some uncommon ones-characteristic of hybridized atomic orbitals, including sp, sp 2 , sp 3 , sp 3 d, sp 3 d 2 , and sp 3 d 3 . Functionalizing the patches with DNA with single-stranded sticky ends makes the interactions between patches on different particles programmable, specific, and reversible, thus facilitating the self-assembly of particles into "colloidal molecules," including "molecules" with triangular, tetrahedral, and other bonding symmetries. Because colloidal dynamics are slow, the kinetics of molecule formation can be followed directly by optical microscopy. These new colloidal atoms should enable the assembly of a rich variety of new micro-structured materials. 2 IntroductionThe past decade has seen an explosion in the kinds of colloidal particles that can be synthesized 1,2 , with many new shapes, such as cubes 3 , clusters of spheres 4-6 and dimpled particles 7,8 reported. Because the self-assembly of these particles is largely controlled by their geometry, only a few relatively simple crystals have been made: face-centered and body-centered cubic crystals and variants 9 . Colloidal alloys increase the diversity of structures [10][11][12] , but many structures remain difficult or impossible to make. For example, the diamond lattice, predicted more than 20 years ago to have a full 3-dimensional photonic band gap 13 , still cannot be made by colloidal self-assembly because it requires 4-fold coordination. Without directional bonds, such low-coordination states are unstable.

DNA-coated colloids hold great promise for self-assembly of programmed heterogeneous microstructures, provided they not only bind when cooled below their melting temperature, but also rearrange so that aggregated particles can anneal into the structure that minimizes the free energy. Unfortunately, DNA-coated colloids generally collide and stick forming kinetically arrested random aggregates when the thickness of the DNA coating is much smaller than the particles. Here we report DNA-coated colloids that can rearrange and anneal, thus enabling the growth of large colloidal crystals from a wide range of micrometre-sized DNA-coated colloids for the first time. The kinetics of aggregation, crystallization and defect formation are followed in real time. The crystallization rate exhibits the familiar maximum for intermediate temperature quenches observed in metallic alloys, but over a temperature range smaller by two orders of magnitude, owing to the highly temperature-sensitive diffusion between aggregated DNA-coated colloids.

This Letter reports the measurement of electro-osmotic flows (EOF ) in microchannels with surface charge patterned on the 200 mm scale. We have investigated two classes of patterns: (1) Those in which the surface charge varies along a direction perpendicular to the electric field used to drive the EOF; this type of pattern generates multidirectional flow along the direction of the field. (2) Those in which the surface charge pattern varies parallel to the field; this pattern generates recirculating cellular flow, and thus causes motion both parallel and perpendicular to the external field. Measurements of both of these flows agree well with theory in the limit of thin double layers and low surface potential. PACS numbers: 47.54. + r, 47.65. + a, 61.20.Lc, 82.45. + z Microfluidic devices are important in a range of applications: microanalytical systems for genomics and proteomics, research tools for cell biology, and low-inventory chemical synthesis. Developments in microfluidics will require sophisticated methods for handling fluids in the low Reynolds number (Re) regime that is imposed by the dimensions of the microchannels in these devices. Electrically driven flow [electro-osmotic flow (EOF)] is an alternative to pressure driven flow in microchannels, but has been used only for the simplest case, that is, for flow driven by uniformly charged channel walls. In this Letter, we describe EOF driven by surface charge patterned on the channel walls [1]. The geometries that we explore demonstrate two new types of fluid behavior at low Reynolds number: multidirectional flow along the axis of the channel and the applied field, and controlled cellular flow with bulk motion both perpendicular and parallel to the axis of the channel and the applied field. We believe that these new types of flows offer opportunities for fine control of fluid motion and chemical transport in microfluidic devices, and also suggest the use of fluids to drive micromechanical elements via shear-generated torques.The experimental challenge of patterning surfaces inside microstructures has hindered the investigation of EOF in the presence of patterned surface charge. For this study, we used two soft lithographic techniques [2] to solve this problem in patterning. We used laminar flow patterning [3] to generate longitudinal patterns, and a modified version of micromolding in capillaries to generate transverse patterns [4]. In both cases, the electrostatic adsorption of organic polymers having charged side groups [poly(ions)][5] onto the inside walls of the channels generated the required regions of positive and negative surface charge, and ensured that these regions had charge densities of similar magnitude [6].Electro-osmotic flow [7] uses an externally applied electric field parallel to a surface with net surface charge density to drive bulk motion in a fluid. The field interacts with the net charge carried by the mobile screening layer (the Debye layer) in the fluid above the surface. A channel with walls having fixed charges thus generates...

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