Benjamin W. Maynor

Generality of shear thickening in dense suspensions

Suspensions are of wide interest and form the basis for many smart fluids. For most suspensions, the viscosity decreases with increasing shear rate, that is, they shear thin. Few are reported to do the opposite, that is, shear thicken, despite the longstanding expectation that shear thickening is a generic type of suspension behaviour. Here we resolve this apparent contradiction. We demonstrate that shear thickening can be masked by a yield stress and can be recovered when the yield stress is decreased below a threshold. We show the generality of this argument and quantify the threshold in rheology experiments where we control yield stresses arising from a variety of sources, such as attractions from particle surface interactions, induced dipoles from applied electric and magnetic fields, as well as confinement of hard particles at high packing fractions. These findings open up possibilities for the design of smart suspensions that combine shear thickening with electro- or magnetorheological response.

Shear thickening and jamming in densely packed suspensions of different particle shapes

We investigated the effects of particle shape on shear thickening in densely packed suspensions. Rods of different aspect ratios and nonconvex hooked rods were fabricated. Viscosity curves and normal stresses were measured using a rheometer for a wide range of packing fractions for each shape. Suspensions of each shape exhibit qualitatively similar discontinuous shear thickening. The logarithmic slope of the stress vs shear rate increases dramatically with packing fraction and diverges at a critical packing fraction φ(c) which depends on particle shape. The packing fraction dependence of the viscosity curves for different convex shapes can be collapsed when the packing fraction is normalized by φ(c). Intriguingly, viscosity curves for nonconvex particles do not collapse on the same set as convex particles, showing strong shear thickening over a wider range of packing fraction. The value of φ(c) is found to coincide with the onset of a yield stress at the jamming transition, suggesting the jamming transition also controls shear thickening. The yield stress is found to correspond with trapped air in the suspensions, and the scale of the stress can be attributed to interfacial tension forces which dramatically increase above φ(c) due to the geometric constraints of jamming. Using this connection we show that the jamming transition can be identified by simply looking at the surface of suspensions. The relationship between shear and normal stresses is found to be linear in both the shear thickening and jammed regimes, indicating that the shear stresses come from friction. In the limit of zero shear rate, normal stresses pull the rheometer plates together due to the surface tension of the liquid below φ(c), but push the rheometer plates apart due to jamming above φ(c).

Microfabricated Engineered Particle Systems for Respiratory Drug Delivery and Other Pharmaceutical Applications

Particle Replication in Non-Wetting Templates (PRINT®) is a platform particle drug delivery technology that coopts the precision and nanoscale spatial resolution inherently afforded by lithographic techniques derived from the microelectronics industry to produce precisely engineered particles. We describe the utility of PRINT technology as a strategy for formulation and delivery of small molecule and biologic therapeutics, highlighting previous studies where particle size, shape, and chemistry have been used to enhance systemic particle distribution properties. In addition, we introduce the application of PRINT technology towards respiratory drug delivery, a particular interest due to the pharmaceutical need for increased control over dry powder characteristics to improve drug delivery and therapeutic indices. To this end, we have produced dry powder particles with micro- and nanoscale geometric features and composed of small molecule and protein therapeutics. Aerosols generated from these particles show attractive properties for efficient pulmonary delivery and differential respiratory deposition characteristics based on particle geometry. This work highlights the advantages of adopting proven microfabrication techniques in achieving unprecedented control over particle geometric design for drug delivery.

Rendering Protein-Based Particles Transiently Insoluble for Therapeutic Applications

Herein, we report the fabrication of protein (bovine serum albumin, BSA) particles which were rendered transiently insoluble using a novel, reductively labile disulfide-based cross-linker. After being cross-linked, the protein particles retain their integrity in aqueous solution and dissolve preferentially under a reducing environment. Our data demonstrates that cleavage of the cross-linker leaves no chemical residue on the reactive amino group. Delivery of a self-replicating RNA was achieved via the transiently insoluble PRINT protein particles. These protein particles can provide new opportunities for drug and gene delivery.

Shear thickening in densely packed suspensions of spheres and rods confined to few layers

We investigate confined shear thickening suspensions for which the sample thickness is comparable to the particle dimensions. Rheometry measurements are presented for densely packed suspensions of spheres and rods with aspect ratios 6 and 9. By varying the suspension thickness in the direction of the shear gradient at constant shear rate, we find pronounced oscillations in the stress. These oscillations become stronger as the gap size is decreased, and the stress is minimized when the sample thickness becomes commensurate with an integer number of particle layers. Despite this confinement-induced effect, viscosity curves show shear thickening that retains bulk behavior down to samples as thin as two particle diameters for spheres, below which the suspension is jammed. Rods exhibit similar behavior commensurate with the particle width, but they show additional effects when the thickness is reduced below about a particle length as they are forced to align; the stress increases for decreasing gap size at fix...

Supramolecular assemblies of DNA with neutral nucleoside amphiphiles

A neutral uridine-based amphiphile is described which condenses plasmid DNA. AFM studies show that the three distinct structural components of the amphiphile (i.e, nucleobase, alkyl chains, and poly(ethylene glycol)) are required for the formation of DNA-amphiphile supramolecular assemblies on a mica surface.

Soft lithography using perfluorinated polyether molds and PRINT technology for fabrication of 3-D arrays on glass substrates

The fabrication of nanometer size structures and complex devices for microelectronics is of increasing importance so as to meet the challenges of large-scale commercial applications. Soft lithography typically employs elastomeric polydimethylsiloxane (PDMS) molds to replicate micro- and nanoscale features. However, the difficulties of PDMS for nanoscale fabrication include inherent incompatibility with organic liquids and the production of a residual scum or flash layer that link features where the nano-structures meet the substrate. An emerging technologically advanced technique known as Pattern Replication in Non-wetting Templates (PRINT) avoids both of these dilemmas by utilizing photocurable perfluorinated polyether (PFPE) rather than PDMS as the elastomeric molding material. PFPE is a liquid at room temperature that exhibits low modulus and high gas permeability when cured. The highly fluorinated PFPE material allows for resistance to swelling by organic liquids and very low surface energies, thereby preventing flash layer formation and ease of separation of PFPE molds from the substrates. These enhanced characteristics enable easy removal of the stamp from the molded material, thereby minimizing damage to the nanoscale features. Herein we describe that PRINT can be operated in two different modes depending on whether the objects to be molded are to be removed and harvested (i.e. to make shape specific organic particles) or whether scum free objects are desired which are adhered onto the substrate (i.e. for scum free pattern generation using imprint lithography). The former can be achieved using a non-reactive, low surface energy substrate (PRINT: Particle Replication in Non-wetting Templates) and the latter can be achieved using a reactive, low surface energy substrate (PRINT: Pattern Replication in Non-wetting Templates). We show that the PRINT technology can been used to fabricate nano-particle arrays covalently bound to a glass substrate with no scum layer. The nanometer size arrays were fabricated using a PFPE mold and a self-assembled monolayer (SAM) fluorinated glass substrate that was also functionalized with free-radically reactive SAM methacrylate moieties. The molded polymeric materials were covalently bound to the glass substrate through thermal curing with the methacrylate groups to permit three dimensional array fabrication. The low surface energies of the PFPE mold and fluorinated glass substrate allowed for no flash layer formation, permitting well resolved structures.

Monodisperse nanocarriers: novel fabrication of polymeric nanoparticles for bio-nanotechnology

The delivery of therapeutic, detection and imaging agents for the diagnosis and treatment of cancer patients has improved dramatically over the years with the development of nano-carriers such as liposomes, micelles, dendrimers, biomolecules, polymer particles, and colloidal precipitates. While many of these carriers have been used with great success in vitro and in vivo, each suffers from serious drawbacks with regard to stability, flexibility, or functionality. To date, there has been no general particle fabrication method available that afforded rigorous control over particle size, shape, composition, cargo and chemical structure. By utilizing the method we has designed referred to as Particle Replication In Non-wetting Templates, or PRINT, we can fabricate monodisperse particles with simultaneous control over structure (i.e. shape, size, composition) and function (i.e. cargo, surface structure). Unlike other particle fabrication techniques, PRINT is delicate and general enough to be compatible with a variety of important next-generation cancer therapeutic, detection and imaging agents, including various cargos (e.g. DNA, proteins, chemotherapy drugs, biosensor dyes, radio-markers, contrast agents), targeting ligands (e.g. antibodies, cell targeting peptides) and functional matrix materials (e.g. bioabsorbable polymers or stimuli responsive matrices). PRINT makes this possible by utilizing low-surface energy, chemically resistant fluoropolymers as molding materials and patterned substrates to produce functional, harvestable, monodisperse polymeric particles.

Molded, High Surface Area Polymer Electrolyte Membranes from Cured Liquid Precursors [J. Am. Chem. Soc. 2006, 128, 12963−12972].

Polymer electrolyte membranes (PEMs) for fuel cells have been synthesized from easily processable, 100% curable, low molecular weight reactive liquid precursors that are photochemically cured into highly proton conductive solid membranes. The liquid precursors were directly cured into membranes of desired dimensions without the need for further processing steps such as melt extrusion or solvent casting. By employing chemical cross-linking, high proton conductivities can be achieved through the incorporation of significant levels of acidic groups without rendering the material water-soluble, which plagues commonly used non-cross-linked polymers. Fabrication of membrane electrode assemblies (MEAs) from these PEMs resulted in fuel cells that outperformed those based on commercial materials. Moreover, these liquid precursors enabled the formation of three-dimensional, patterned PEMs with high fidelity, micron-scale features by using soft lithographic/micromolding techniques. The patterned membranes provided a larger interfacial area between the membrane and catalyst layer than standard flat PEMs. MEAs composed of the patterned membranes demonstrated higher power densities over that of flat ones without an increase in the macroscopic area of the fuel cells. This can potentially miniaturize fuel cells and promote their application in portable devices.