Micro-transfer patterning of dense nanoparticle layers: roles of rheology, adhesion and fracture in transfer dynamics.

Liquid inks deposited on substrates undergo spreading, coalescence, dewetting and subsequent drying kinetics, which limit the controllability of the cross-sectional shape and resolution of the printed patterns. By contrast, when the ink layers are previously semidried (highly-concentrated) and patterned on a polydimethylsiloxane sheet, single-micrometer features are resolved. Here we present the rheological, fracture and adhesive properties of semidried nanoparticle dispersion ink layers, which optimize the patterning of reverse offset printing with 5 µm spatial resolution. Under the appropriate patterning conditions, when the volume fraction φ of the particles in the semidried layers was approximately 46 v/v%, the layer elasticity was dominant in the linear viscoelastic region and a Burgers-type creeping property appeared. Under tensile strain, the semidried layers suddenly fractured at the sites of patterns with sharply defined sidewalls. In the semidried thin layers dominated by viscosity (lower φ), the pattern edges were degraded owing to local transfer instability and possible subsequent spreading. Over-drying reduced the adhesiveness of the ink layers, implying an upper limit of φ for successful patterning. The characteristics of semidried inks contribute to establishing a versatile ink-formulation scheme of various functional nanomaterials for high-resolution printed applications.

Forbidden directions for the fracture of thin anisotropic sheets: an analogy with the Wulff plot.

It is often postulated that quasistatic cracks propagate along the direction allowing fracture for the lowest load. Nevertheless, this statement is debated, in particular for anisotropic materials. We performed tearing experiments in anisotropic brittle thin sheets that validate this principle in the case of weak anisotropy. We also predict the existence of forbidden directions and facets in strongly anisotropic materials, through an analogy with the description of equilibrium shapes in crystals. However, we observe cracks that do not necessarily follow the easiest direction but can select a harder direction, which is only locally more advantageous than neighboring paths. These results challenge the traditional description of fracture propagation, and we suggest a modified, less restrictive criterion compatible with our experimental observations.

Capillary motor driven by electrowetting.

A micro-structure supported on a droplet is subjected to capillary force and aligned dependent on its shape. If the droplet's boundary conditions at the bottom and the micro-structure are non-circular, capillary torque is exerted on the structures. The direction of torque is determined by the boundary conditions and the position of the structure. By changing the boundary conditions continuously, rotational motion of a plate was achieved. The boundary conditions of the droplet were controlled by electrowetting. We patterned electrodes in an annular shape on the plate supporting the droplet. By changing the voltage-applied electrodes, the boundary conditions were changed and the plate is rotated. The droplet and the plate worked as a capillary motor with this method. We report the relationship between the characteristics of the capillary motor and its rotational motion. We sandwiched a 3.0-microL water droplet between two plates and achieved a rotational motion of 720 rpm at maximum.

Capillary torque caused by a liquid droplet sandwiched between two plates.

Capillary force makes a liquid droplet's surface have the minimum area. If the droplet is sandwiched between two plates, it exerts capillary force on the plates. The magnitude of the force depends on the shape of the sandwiched droplet, which is in turn determined by the shape of the plates and the volume of the liquid. The liquid's shape, however, is hard to determine analytically. In this paper, the torque caused by a droplet sandwiched between two noncircular plates is experimentally and theoretically analyzed. We patterned a magnetic material on the surface of the plates and used it to apply a magnetic force to the plates. The torque on the plates was measured. The torque caused by capillary force was calculated by observing the equilibrium between the capillary force and magnetic force. We obtained approximate theoretical solutions for the liquid's shape and torque and verified that they were in accordance with the experimental results. The experimental and theoretical results presented in this paper are useful for designing microdevices or self-assemblies actuated by capillary force.

Mechanisms of Adhesive Micropatterning of Functional Colloid Thin Layers.

Thin-film layers of nanoparticles exhibit mechanical fragility that depends on their interactions. Balancing the cohesive force of particles with their interfacial adhesion to a substrate enables the selective transfer of micrometer-scale layer features. Here, the versatility of this adhesion-based transfer approach from poly(dimethylsiloxane) (PDMS) is presented by demonstrating micropatterns of various functional nanoparticulate materials, including Ag, Cu, indium tin oxide, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and dielectric silica. With the attachment of the Johnson-Kendall-Roberts interaction to a simple strain model of particle layers during the patterning process, the patterning criteria for successful printing at both macroscale and nanoscale levels are deduced. Discrete element modeling analysis was used to validate the scaling laws and to highlight the fracture modes of particle layers during the patterning process. In particular, the balance among cohesive forces in the tensile direction and in the shear direction and the adhesion force at the layer-PDMS interface mainly regulates the patterning quality of adhesion patterning.

High-Aspect-Ratio Ridge Structures Induced by Plastic Deformation as a Novel Microfabrication Technique.

Wrinkles on thin film/elastomer bilayer systems provide functional surfaces. The aspect ratio of these wrinkles is critical to their functionality. Much effort has been dedicated to creating high-aspect-ratio structures on the surface of bilayer systems. A highly prestretched elastomer attached to a thin film has recently been shown to form a high-aspect-ratio structure, called a ridge structure, due to a large strain induced in the elastomer. However, the prestretch requirements of the elastomer during thin film attachment are not compatible with conventional thin film deposition methods, such as spin coating, dip coating, and chemical vapor deposition (CVD). Thus, the fabrication method is complex, and ridge structure formation is limited to planar surfaces. This paper presents a new and simple method for constructing ridge structures on a nonplanar surface using a plastic thin film/elastomer bilayer system. A plastic thin film is attached to a stress-free elastomer, and the resulting bilayer system is highly stretched one- or two-dimensionally. Upon the release of the stretch load, the deformation of the elastomer is reversible, while the plastically deformed thin film stays elongated. The combination of the length mismatch and the large strain induced in the elastomer generates ridge structures. The morphology of the plastic thin film/elastomer bilayer system is experimentally studied by varying the physical parameters, and the functionality and the applicability to a nonplanar surface are demonstrated. Finally, we simulate the effect of plasticity on morphology. This study presents a new technique for generating microscale high-aspect-ratio structures and its potential for functional surfaces.

Ridge localizations and networks in thin films compressed by the incremental release of a large equi-biaxial pre-stretch in the substrate.

Two-dimensional ridge structures are induced by equi-biaxial compression with large equi-biaxial pre-stretch in a thin film by using a micro-fluidics technique. Whereas wrinkles tend to be uniformly distributed, ridges are localized. The wrinkle-to-ridge transition is unstable (subcritical), resulting in large amplitude changes. The nature and morphology of the ridges is studied and quantified by experiments and numerical simulations.