When a spilled drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter (Fig 1a) The coffee—initially dispersed over the entire drop—becomes concentrated into a tiny fraction of it Such ring deposits are common wherever drops containing dispersed solids evaporate on a surface, and they influence processes such as printing, washing and coating1,2,3,4,5 Ring deposits also provide a potential means to write or deposit a fine pattern onto a surface Here we ascribe the characteristic pattern of the deposition to a form of capillary flow in which pinning of the contact line of the drying drop ensures that liquid evaporating from the edge is replenished by liquid from the interior The resulting outward flow can carry virtually all the dispersed material to the edge This mechanism predicts a distinctive power-law growth of the ring mass with time—a law independent of the particular substrate, carrier fluid or deposited solids We have verified this law by microscopic observations of colloidal fluids

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Capillary flow as the cause of ring stains from dried liquid drops

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micro...

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Forming regular textures of an arbitrary size on smooth solid surfaces is the challenge of future technology to produce new types of optical gratings, optical filters, antireflective surface coatings, selective solar absorbers, data storage, and microelectronics. Here we present a novel approach to form such sophisticated textures:  controlling the growth of particle arrays on smooth and wettable solid surfaces. The obtained centimeter-size polycrystalline monolayer films consist of closely packed fine particles. Coloring of the monolayer which arises from the light diffraction, interference, and scattering exclusively inherent in textured films shows the size of the differently oriented crystal domains building the film. The results show that the higher the particle monodispersity, the lower the particle volume fraction, and the higher the environmental humidity, the larger the size of the forming domains.

Continuous Convective Assembling of Fine Particles into Two-Dimensional Arrays on Solid Surfaces

The synthesis of highly ordered macroporous materials has been accomplished in a straightforward, single-step reaction. Inorganic frameworks composed of oxides of Si, Ti, Zr, Al, W, Fe, Sb, and a Zr/Y mixture were formed from metal alkoxide precursors templated around polystyrene (latex) spheres. Monodisperse latex spheres were ordered into close-packed arrays by centrifugation. The interstices between latex spheres were permeated by the alkoxide, which hydrolyzed and condensed. An inorganic framework was formed upon drying. Removal of the latex spheres was accomplished by either calcination at temperatures between 450 and 1000 °C or extraction with a tetrahydrofuran/acetone mixture. The resulting products consisted of periodic, interconnected networks of monodisperse submicron pores extending over hundreds of micrometers. Depending on the technique of template removal, various phases of the inorganic oxide could be formed. For example, in the case of titania, an amorphous phase was formed upon extraction...

Synthesis of highly ordered, three-dimensional, macroporous structures of amorphous or crystalline inorganic oxides, phosphates, and hybrid composites

https://www.lcpe.uni-sofia.bg/publications/2000/pdf/2000-01-PK.pdf

Capillary interactions between particles bound to interfaces, liquid films and biomembranes.

The dynamics of two-dimensional ordering of micrometer-size polystyrene latex spheres on a horizontal glass substrate has been directly observed by means of optical microscopy. It turns out that the ordering starts when the thickness of the water layer containing particles becomes approximately equal to the particle diameter. By variation of the electrolyte concentration, the charge of the particles, and their volume fraction, it is proven that neither the electrostatic repulsion nor the van der Waals attraction between the particles is responsible for the formation of two-dimensional crystals. The direct observations revealed the main factors governing the ordering-the attractive capillary forces (due to the menisci formed around the particles) and the convective transport of particles toward the ordered region. The control of the water evaporation rate turns out to allow obtaining either well-ordered monolayers or well-ordered domains consisting of multilayers (bilayers, trilayers, etc.).

https://www.lcpe.uni-sofia.bg/publications/1992/pdf/1992-01-PK.pdf

P. Kralchevski

Papers

Mechanism of formation of two-dimensional crystals from latex particles on substrates