Our Research

The principal activites of the Lewis Group involve the directed and self-assembly of soft materials. By bringing together our expertise in colloidal science, rheology, drying, microfluidics, and direct writing approaches, we are designing functional materials with controlled composition and architecture on multiple length scales. Specifically, we are creating novel materials that may find potential applications as ceramics, photonics, sensors, tissue engineering scaffolds, and microvascular networks. Our group is divided into three main sub-groups with a rich and overlapping set of interests:

(1) Complex Fluids - We investigate the phase behavior, structure, and rheology of colloidal suspensions using a broad array of techniques, including light scattering, rheological measurements, in situ drying stress measurements, and direct visualization approaches, such as confocal microscopy and high speed imaging. Our current focus includes microsphere-nanoparticle mixtures, biphasic colloid mixtures, colloid-filled hydrogels, polyelectrolyte complexes, and photoresponsive colloid systems.

(2) Colloidal Assembly - We employ directed assembly approaches including colloidal epitaxy, evaporative lithography, and microfluidic devices to create precisely patterned colloidal films, granules, and other 3D forms.

(3) Direct Ink Writing - We are designing novel inks for direct-write assembly of planar and 3-D structures with locally tailored composition and architecture. A myriad of ink designs are under development, including colloidal, nanoparticle, fugitive organic, polyelectrolyte and sol-gel inks. Complex 3D structures h ave been produced with minimum feature sizes ranging from ~ 0.2 µm to 300 µm

Our work is funded principally by the Department of Energy, National Science Foundation, Army Research Office MURI program, Air Force Office of Scientific Research MURI program, NASA, and has benefited from fruitful collaborations with Sandia National Laboratories and industry.

Cover Articles

February 2010: Direct-write assembly of biomimetic microvascular networks for efficient fluid transport

Biomimetic microvascular networks with complex architectures are embedded in epoxy matrices using direct-write assembly. Fluid transport in multi-generation bifurcating channels is systematically investigated and maximum flow efficiency is found to occur when Murray's law is obeyed.

December 2009: Two- and three-dimensional folding of thin film single-crystalline silicon for photovoltaic power applications

Fabrication of 3D electronic structures in the micrometer-to-millimeter range is extremely challenging due to the inherently 2D nature of most conventional wafer-based fabrication methods. Self-assembly, and the related method of self-folding of planar patterned membranes, provide a promising means to solve this problem. Here, we investigate self-assembly processes driven by wetting interactions to shape the contour of a functional, nonplanar photovoltaic (PV) device. A mechanics model based on the theory of thin plates is developed to identify the critical conditions for self-folding of different 2D geometrical shapes. This strategy is demonstrated for specifically designed millimeter-scale silicon objects, which are self-assembled into spherical, and other 3D shapes and integrated into fully functional light-trapping PV devices. The resulting 3D devices offer a promising way to efficiently harvest solar energy in thin cells using concentrator microarrays that function without active light tracking systems.

September 2009: Silk Fibroin Waveguides: Biocompatible Silk Printed Optical Waveguides

Biocompatible silk optical waveguides are fabricated by direct-write assembly and demonstrated to guide light in both straight and curved architectures. These waveguides can easily be doped or functionalized with bioactive molecules, and are promising materials for biophotonic devices.

December 2008: Stop-Flow Lithography of Colloidal, Glass, and Silicon Microcomponents

The assembly of oxide and non-oxide microcomponents from colloidal building blocks is central to a broad array of applications, including sensors, optical devices, and microelectromechanical systems (MEMS), as well as to fundamental studies of granular materials. Progress in these areas has been hindered by the availability of colloidal microcomponents of precisely size, shape, and composition. Hence, there is tremendous interest in developing new patterning methods for creating preciesely tailored microcomponents composed of colloidal building blocks, amorphous or polycrystalline oxides, and silicon.

October 2008: Ultrathin silcon solar microcelles for semitransparent, mechanically flexible and microconcentrator module designs

The high natural abundance of silicon, together with its excellent reliability and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise, many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems.Here,we describemodules that use large-scale arrays of silicon solar microcells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable transparency and ultrathin-form-factor microconcentrator designs. Detailed studies of the processes for creating and manipulating such microcells, together with theoretical and experimental investigations of the electrical, mechanical and optical characteristics of several types of module that incorporate them, illuminate the key aspects.

June 2007: Sol-Gel Inks for Direct-Write Assembly of Functional Oxides

Germanium inverse woodpile 3D photonic crystals with a large (25%) photonic band gap in the infrared (background image) were fabricated through a multistep replication procedure. A polymer scaffold was first created by direct-write assembly, followed by the conformal growth of oxide and semiconductor layers, and removal of the polymer and oxide (foreground), as reported on p. 1567 by Paul Braun, Jennifer Lewis, and co-workers.


December 2006: Direct Ink Writing of Three-Dimensional Ceramic Structures

The ability to pattern ceramic materials in three dimensions (3D) is critical for structural, functional, and biomedical applications. One facile approach is direct ink writing (DIW), in which 3D structures are built layer-by-layer through the deposition of colloidal- or polymer-based inks. This approach allows one to design and rapidly fabricate ceramic materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. In this feature article, we present both dropletand filament-based DIW techniques. We focus on the various ink designs and their corresponding rheological behavior, ink deposition mechanics, potential shapes and the toolpaths required, and representative examples of 3D ceramic structures assembled by each technique. The opportunities and challenges associated with DIW are also highlighted.

November 2006: Direct Ink writing of 3D Functional Materials

The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.

January 2007: Phase Behavior, 3-D Structure, and Rheology of Colloidal Microsphere-Nanoparticle Suspensions

A new route for tailoring the behavior of colloidal suspensions through nanoparticle additions is reviewed. Specifically, the interparticle interactions, phase behavior, 3-D structure, and rheological properties of microsphere-nanoparticle mixtures that possess both high charge and size asymmetry are described. Negligibly charged microspheres, which flocculate when suspended alone, undergo a remarkable stabilizing transition upon the addition of highly charged nanoparticles. The formation of a dynamic nanoparticle halo around each colloid induces an effective repulsion between the microspheres that promotes their stability. With increasing nanoparticle concentration, the colloids again undergo flocculation because of the emergence of an effective microsphere attraction, whose magnitude exhibits a quadratic dependence on nanoparticle volume fraction. The broader impact of these observations on colloidal stabilization and assembly of advanced ceramics is highlighted.

February 2006: Biomimetic silicification of 3D polyamine-rich scaffolds assembled by direct ink writing

We report a method for creating synthetic diatom frustules via the biomimetic silicification of polyamine-rich scaffolds assembled by direct ink writing (DIW) [G. M. Gratson, M. Xu and J. A. Lewis, Nature, 2004, 428, 386, ref. 1]. A concentrated polyamine-rich ink is robotically deposited in a complex 3D pattern that mimics the shape of naturally occurring diatom frustules, Triceratium favus Ehrenberg (triangular-shaped) and Arachnoidiscus ehrenbergii (webshaped). Upon exposing these scaffolds to silicic acid under ambient conditions, silica formation occurs in a shapepreserving fashion. Our method yields 3D inorganic-organic hybrids structures that may find potential application as templates for photonic materials, novel membranes, or catalyst supports.

2005 - Cellular Processing of Ceramics

Cellular ceramics are a specific class of porous materials which includes among others foams, honeycombs, connected fibers, robocast structures and assembled hollow spheres. Because of their particular structure, cellular ceramics display a wide variety of specific properties which make them indispensable for various engineering applications. An increasing number of patents, scientific literature and international conferences devoted to cellular materials testifies to a rapidly growing interest of the technical community in this topic. New applications for cellular ceramics are constantly being put under development.

The book, authored by leading experts in this emerging field, gives an overview of the main aspects related to the processing of diverse cellular ceramic structures, methods of structural and properties characterisation and well established industrial, novel and potential applications. It is an introduction to newcomers in this research area and allows students to obtain an in-depth knowledge of basic and practical aspects of this fascinating class of advanced materials.

August 2004: Direct Writing in Three Dimensions

The ability to pattern materials in three dimensions is critical for several emerging technologies, including photonics, microfluidics, microelectromechanical systems, and biomaterials. Direct-write assembly allows one to design and rapidly fabricate materials in complex three-dimensional shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in ink and laser writing techniques are reviewed with an emphasis on the push toward finer feature sizes. Opportunities and challenges associated with direct-write assembly are also highlighted.

August 2004: Fabricated Microvascular Networks

Under funding provided by the Air Force Office of Scientific Research and the National Science Foundation, Dr. Scott White, a professor of aerospace engineering at the University of Illinois at Urbana-Champaign (UIUC) and a Willett Faculty Scholar at the Beckman Institute for Advanced Sciences and Technology, teamed with other UIUC scientists to discover a technique for fabricating three-dimensional microvascular networks. These miniscule networks could have many uses as compact fluidic channels and reservoirs in sensors, chemical reactors, and computers. Dr. White collaborated with Dr. Jennifer Lewis, a UIUC professor of material science and chemical engineering, and Mr. Daniel Therriault, a UIUC graduate student, to produce a pervasive network of interconnected cylindrical channels that can range from 10 to 300 µm in diameter.

February 2004: Nanoparticle-Mediated Epitaxial Assembly of Colloidal Crystals on Patterned Structures

We have studied the assembly of 3-D colloidal crystals from binary mixtures of colloidal microspheres and highly charged nanoparticles on flat and epitaxially patterned substrates created by focused ion beam milling. The microspheres were settled onto these substrates from dilute binary mixtures. Laser scanning confocal microscopy was used to directly observe microsphere structural evolution during sedimentation, nanoparticle gelation, and subsequent drying. After microsphere settling, the nanoparticle solution surrounding the colloidal crystal was gelled in situ by introducing ammonia vapor, which increased the pH and enabled drying with minimal microsphere rearrangement. By infilling the dried colloidal crystals with an index-matched fluorescent dye solution, we generated full 3-D reconstructions of their structure including defects as a function of initial suspension composition and pitch of the patterned features. Through proper control over these important parameters, 3-D colloidal crystals were created with low defect densities suitable for use as templates for photonic crystals and photonic band gap materials. (research carried out in collaboration with the Braun Group)

December 2002 - Chemistry Highlights

Jennifer A. Lewis of UIUC and co-workers used colloidal gels (inks) to construct intricate 3-D structures with micrometer-size features and overall dimensions of a few millimeters [Langmuir, 18, 5429 (2002); C&EN, July 1, page 7]. Possible uses include advanced ceramics, photonic materials, and catalyst supports.

May 2002: Colloidal Inks for Directed Assembly of 3-D Periodic Structures

Mesoscale periodic structures have been fabricated via directed assembly of colloidal inks. Concentrated colloidal gels with tailored viscoelastic properties were designed to form self-supporting features. The inks were deposited in a layer-by-layer sequence to directly write the desired 3-D pattern. Periodic structures with spanning features that vary between 100 µm and 1 mm were assembled. Shear rate profiles were calculated on the basis of the measured rheological properties of the inks under slip and no-slip boundary conditions during flow through a cylindrical deposition nozzle. Deflection measurements of spanning elements were used to probe the relationship between gel strength, deposition speed, and shear rate profiles in the nozzle. These observations revealed that the ink adopted a rigid (gel) core-fluid shell architecture during assembly, which simultaneously facilitated bonding and shape retention of the deposited elements.

May 2001: Nanoparticle Halos: A New Colloid Stabilization Mechanism

A new mechanism for regulating the stability of colloidal particles has been discovered. Negligibly charged colloidal microspheres, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microspheres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these nanoparticle species did not adsorb on the microspheres under the experimental conditions of interest. It is therefore proposed that highly charged nanoparticles segregate to regions near negligibly charged microspheres because of their repulsive Coulombic interactions in solution. This type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids.

June 2000: Colloidal Processing of Ceramics

Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.