When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect, is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids to nanoparticles and individual molecules. In fact--despite the many practical applications for uniform coatings in printing, biology and complex assembly-the ubiquitous nature of the effect has made it difficult to avoid. Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions. Thus, after the ellipsoids are carried to the air-water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air-water interface. These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.

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Suppression of the coffee-ring effect by shape-dependent capillary interactions

This Review summarizes recent developments in the field of responsive photonic crystal structures, including principles for design and fabrication and many strategies for applications, for example as optical switches or chemical and biological sensors. A number of fabrication methods are now available to realize responsive photonic structures, the majority of which rely on self-assembly processes to achieve ordering. Compared with microfabrication techniques, self-assembly approaches have lower processing costs and higher production efficiency, however, major efforts are still needed to further develop such approaches. In fact, some emerging techniques such as spin coating, magnetic assembly, and flow-induced self-assembly have already shown great promise in overcoming current challenges. When designing new systems with improved performance, it is always helpful to bear in mind the lessons learnt from natural photonic structures.

Responsive photonic crystals.

Inkjet printing is an attractive patterning technology, which has become increasingly accepted for a variety of industrial and scientific applications. This review primarily presents an overview of the investigations that have been conducted since 2003 into inkjet-printing polymers or metal-containing inks and mentions related activities. The first section discusses the droplet-formation process in piezoelectric drop-on-demand printheads and the physical properties that affect droplet formation and the resolution of inkjet-printed features. The second section deals with the issues that arise from printing polymers, such as printability and the output characteristics of devices made by this route. Finally, the challenges and achievements attached to inkjet printing metal-containing inks are discussed before concluding with a few remarks about the future of the field.

Inkjet printing as a deposition and patterning tool for polymers and inorganic particles

Particle deposit morphologies that resulted from evaporating ink-jetted microdroplets were controlled by varying the ink compositions and concentrations. The ink was a well-dispersed aqueous dispersion of monodisperse silica microspheres. Silica particles suspended in the microdroplet undergo self-assembly upon the evaporation of the solvent. A ringlike deposit of the self-assembled silica particles was produced from the water-based ink, while a uniform two-dimensional monolayer with a well-ordered hexagonal structure was obtained from the mixed-solvent-based inks. Variations in the deposit patterns can be explained in terms of competing effects between the convective and Marangoni flows, which vary with the types of the high-boiling-point solvent added to the ink. The macroscopic shape and microstructure of the silica colloidal deposits were observed by SEM, AFM, and a confocal microscope.

Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing.

This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions

Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions.

An array of hemispherical particle assemblies with ordered nanoporous structures is fabricated by ink-jet printing of colloidal ink on a hydrophobic solid surface. The ordered structure is generated by evaporating the solvent of the colloidal ink, which induces monodisperse silica particles self-assembled within a droplet template.

Rapid Self-Assembly of Monodisperse Colloidal Spheres in an Ink-Jet Printed Droplet

Direct-write fabrication of colloidal photonic crystal microarrays by ink-jet printing

Systematic investigations on the microstructural evolution of a bimodal eutectic (Ti705Fe295)91Sn9 ultrafine composite containing Ti3Sn dendrites upon compression reveal that local deformation of the dendrites dominates the early stage of deformation with a plastic strain of ep=58% After further deformation (ep=102%), a wavy propagation of shear bands indicative of dissipation of the shear stress is caused by a rotation of the coarse eutectic structure along the interfaces of the bimodal eutectic structure

Influence of a bimodal eutectic structure on the plasticity of a (Ti70.5Fe29.5)91Sn9 ultrafine composite

An array of droplets of photonic crystal with polystyrene colloidal particles was assembled on silicon using ink-jet printing. A microreflectance spectrum from a single droplet was collected using an optical microscope and a spectrometer. It was observed that the photonic band gap of these droplets shifts as a function of the size of the colloidal particle. The reflectance peak position predicted from the model based on Bruggeman and Maxwell–Garnett effective medium approximations agrees well with those of the experimentally observed peak.

Jungho Park

Papers

Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing.

Rapid Self-Assembly of Monodisperse Colloidal Spheres in an Ink-Jet Printed Droplet

Direct-write fabrication of colloidal photonic crystal microarrays by ink-jet printing

Influence of a bimodal eutectic structure on the plasticity of a (Ti70.5Fe29.5)91Sn9 ultrafine composite

Optical properties of single droplet of photonic crystal assembled by ink-jet printing