Designing colloidal suspensions for directed materials assembly

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o Recent advances in microfluidic and direct-write assembly of colloidal suspensions open new avenues for microscale patterning of functional materials with controlled composition, geometry, and properties. In this article, we describe fundamental aspects of suspension structure, elasticity, and flow behavior for three important systems: (1) dense liquids (and glasses), (2) gels, and (3) biphasic mixtures. We also highlight examples from the current literature of colloidal architectures patterned by these emerging methods. The ability to pattern colloidal suspensions in planar and three-dimensional motifs is critical for myriad technologies, including optical displays, designer pharmaceuticals, micro-electromechanical systems, and printed electronics. The broad diversity of potentially relevant materials, length scales, and architectures underscores the need for flexible patterning approaches. Emerging routes based on microfluidic [1 • ,2 • • ] and direct-write assembly [3 • • ] enable the rapid design and fabrication of colloidal materials without the need for expensive tooling, dies, or lithographic masks, and therefore offer exciting opportunities for materials assembly. Colloidal suspensions used in these directed assembly methods must flow without clogging through confined geometries [4], such as microchannel(s) or nozzle(s) within the fluidic device or printhead. The desired flow behavior can be achieved by carefully controlling three key parameters: the colloid volume fraction, ϕ, interparticle attraction strength, U, and applied stress, σ, as shown in Fig. 1a [5 • • ]. These parameters also strongly influence the structure and elasticity of colloidal suspensions, which enable patterned structures to retain their shape and solidity after forming. In this article, we first describe three types of dense colloidal suspensions that can be designed for the microfluidic and direct-write assembly methods of interest: dense fluids and glasses, gels, and biphasic mixtures. Next, we introduce examples from the current literature of patterned colloidal architec-tures, ranging from novel granular media to flexible, stretchable, and spanning microelectrodes for printed electronics. Finally, opportunities and challenges associated with these emerging methods are highlighted. 2. Designing colloidal suspensions 2.1. Suspension structure The design of colloidal suspensions for microfluidic and direct-write assembly requires a fundamental understanding of the relationship between suspension …