Suspensions are of wide interest and form the basis for many smart fluids [1][2][3][4][5][6][7]. For most suspensions, the viscosity decreases with increasing shear rate, i.e. they shear thin. Few are reported to do the opposite, i.e. shear thicken, despite the longstanding expectation that shear thickening is a generic type of suspension behavior [8, 9]. 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 is presumed to be due to general mechanisms such as hydrodynamics [9, 10] or dilation [11][12][13], and thus all suspensions are expected to exhibit shear thickening under the right conditions [8]. So far, however, the exact conditions have not been determined. One condition is apparently set by attractive particle interactions. It has long been known that attractions, observed as flocculation in suspensions, can prevent shear thickening. This has been shown by modifying the chemistry, for example by adding flocculating agents to observe the transition from shear thickening to thinning (for a review, see [8]). In other cases, crossing the gel transition was shown to eliminate shear thickening [14,15]. A key problem, therefore, is to understand how interparticle attractions interfere with shear thickening. We demonstrate here that a simple and direct criterion for the existence of an observable shear thickening regime in dense, non-Brownian suspensions can be developed by comparing the yield stress produced by attractions with the inherent shear thickening stresses. We then generalize this condition to show how a yield stress from any source modifies the shear thickening phase diagram.Our experiments used an Anton Paar rheometer to * Electronic address: embrown@uchicago.edu •: viscosity curve of the same system at the same φ with added surfactant. The shear thickening regime is the region of positive slope in the curves of viscosity η versus applied stress τ . Shear thinning is characterized by a negative slope and Newtonian fluids, such as water, exhibit constant η. b, Images show clustering due to interparticle attractions (top) and no clustering when surfactant is added (bottom). Scale bar is 200 µm. All images (including subsequent figures) were taken at rest under an optical microscope in a dilute quasi two-dimensional layer. In this dilute case, attractions can be observed by the high number of particle contacts in the form of clusters or chains.measure the shear stress τ an...

(NMR) analysis, using the tris [3-(trifluoromethylhydroxymethylene)-(ϩ)camphorato]europium(III) complex. The compound was hydrolyzed to the corresponding acid and then the optical purity was enhanced up to 97% ee by cocrystallization with commercially available R-PEA. The optical purity was checked by NMR and circular dichroism spectra:[␣] D 25 ϭ Ϫ63.2 (C ϭ 1.01 g/liter tetrahydrofuran). 5. The isotherms were measured from A ϭ 100 to 5 Å 2

The dispersion polymerization of styrene in supercritical CO2 using amphiphilic diblock copolymers to impart steric stabilization was investigated. Lipophilic, CO2-insoluble materials can be effectively emulsified in carbon dioxide using amphiphilic diblock copolymer surfactants. The resulting high yield (>90%) of polystyrene is obtained in the form of a stable polymer colloid comprised of submicron-sized particles. The particle diameter and distribution of sizes were shown to be dependent on the nature of the stabilizing block copolymer.

X-ray photoelectron spectroscopy (XPS) has been used to investigate the surface characteristics of various novel fluorinated acrylate homopolymers [1,1-dihydroperfluorooctyl acrylate (PFOA), 1,1-dihydroperfluorooctyl methacrylate (PFOMA), 1,1,2,2-tetrahydroperfluorooctyl acrylate (PTAN)] as well as diblock copolymers consisting of both a fluorocarbon block of PFOA and a hydrocarbon block of polystyrene (PS). This technique allows nondestructive depth profiling of the top ∼100 Å of a material, providing both elemental composition and chemical state information. Due to the low surface energy of the fluorinated species, its enhanced presence on the surface is of importance in any potential applications. Angle-dependent XPS surface studies were conducted on polymer thick films to monitor surface segregation of the fluorinated component as a function of depth. Fluorine and the fluorine-containing constituents are surface enriched relative to carbon and oxygen from the acrylate portions of the polymers. This effect also occurs in the diblock copolymers, where the PFOA block prefers the polymer−air interface. Furthermore, this surface segregation is enhanced when the samples are thermally annealed. Also, the quantitative XPS data reveal other subtleties in the overall polymer structures, such as extent of chain branching in PFOA, PFOMA, and the diblock copolymers and the slight variations in average fluorine-containing side chain lengths in PTAN.

We demonstrate a facile way of cross-linking hydrophobic perfluoropolyethers, PFPEs, with a series of hydrophilic poly(ethylene glycol)s, PEGs, to prepare a range of amphiphilic networks for use as fouling-release coatings. The PFPE matrix of the networks endows the coating with a low surface energy while the PEG is added to weaken fouling adhesion. It is therefore envisioned that the coating surfaces of these optically transparent and mechanically robust films will display hydrophobicity leading to nonfouling and fouling release characteristics. Two kinds of functionalized PEG oligomers have been cross-linked with reactive, dimethacryloxy-functionalized PFPE oligomers to form a range of amphiphilic networks: (i) a monomethacryloxy-functionalized PEG macromonomer (454 g/mol) (PEG454-MA) which was used to yield blends with flexible PEG chains on the surface as well as in bulk and (ii) a dimethacryloxy-functionalized PEG (550 g/mol) (PEG550-DMA) which results in PEG chains that are relatively more restricted in the network blends and serve as an added difunctional cross-linker for the network along with the dimethacryloxy-functionalized PFPE. The PFPE/PEG cross-linked networks coated on a substrate show very low swelling characteristics in water when PEG454-MA comprises not more than 10 wt % of the overall composition or when PEG550-DMA is used and does not comprise more than 30 wt % of the overall composition. The PFPE/PEG454-MA coatings having PEG chains with one untethered chain end were found to display relatively high spore and barnacle release performance in comparison to PFPE/PEG550-DMA coatings which have the PEG chains in a more restricted network topology.

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).

Amphiphilic networks of perfluoropolyethers (PFPE) and poly(ethylene glycol) (PEG) have been achieved to yield optically transparent, mechanically robust films over a wide range of compositions. Telechelic diols of these oligomers were transformed to a photocurable dimethacryloxy form (DMA) and free radically cured at various composition weight ratios to yield free-standing films. Clear and colorless amphiphilic networks could be achieved when low molar mass versions of both the PFPE-DMA (1 kg/mol) and the PEG-DMA (550 g/mol) were used. The bulk morphologies of the samples were extensively characterized by a variety of techniques including ultraviolet-visible spectroscopy, differential scanning calorimetry, dynamic mechanic thermal analysis, small-angle X-ray scattering, atomic force microscopy, X-ray photoelectron spectroscopy, and optical microscopy, which strongly suggest that nanoscopic to macroscopic phase-separated materials could be achieved. By incorporating a threshold amount of PFPEs into PEG-based hydrogel networks, water swelling could be significantly reduced, which may offer a new strategy for a number of medical device applications. Along these lines, strong inhibition of nonspecific protein adsorption could be achieved with these amphiphilic network materials compared with an oligo(ethylene glycol)-based self-assembled monolayer coated surface.

A series of reactive liquid perfluoropolyether (PFPE) precursors were synthesized which can be photochemically cross-linked (UV-cured) into high-performance PFPE elastomers in one step. To investigate how fundamental changes in the PFPE molecular structure correlate to bulk and surface properties, the variable functional end group, molecular weight, and the copolymer content were systematically explored in relation to thermal stability, contact angle/surface tension, modulus, and biofouling behavior. The morphologies of these PFPE materials were studied using differential scanning calorimetry, dynamic mechanical thermal analysis, and small-angle X-ray scattering. From these studies, it was determined that clusters of polymerized functional end groups were found to be nanophase separated within the PFPE matrix. By varying the cross-link density, the Young’s modulus of the fully cross-linked PFPE elastomeric film could be tuned from 1.5 to 90 MPa with a critical surface tension of 8.6−16 mN/m. The marine antifouling and fouling-release properties of the cross-linked PFPE elastomeric coatings were evaluated by settlement and release assays involving zoospores and sporelings (young plants), respectively, of green fouling alga Ulva.