A versatile "top-down" method for the fabrication of particles, Particle Replication In Nonwetting Templates (PRINT), is described which affords absolute control over particle size, shape, and composition. This technique is versatile and general enough to fabricate particles with a variety of chemical structures, yet delicate enough to be compatible with sophisticated biological agents. Using PRINT, we have fabricated monodisperse particles of poly(ethylene glycol diacrylate), triacrylate resin, poly(lactic acid), and poly(pyrrole). Monodisperse particle populations, ranging from sub-200 nm nanoparticles to complex micron-scale objects, have been fabricated and harvested. PRINT uses low-surface energy, chemically resistant fluoropolymers as molding materials, which eliminates the formation of a residual interconnecting film between molded objects. Until now, the presence of this film has largely prevented particle fabrication using soft lithography. Importantly, we have demonstrated that PRINT affords the simple, straightforward encapsulation of a variety of important bioactive agents, including proteins, DNA, and small-molecule therapeutics, which indicates that PRINT can be used to fabricate next-generation particulate drug-delivery agents.
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...
A new technique for direct-writing of polymer nanostructures on insulating and semiconducting surfaces based on Electrochemical Dip-Pen Nanolithography (E-DPN) is described. The technique is based on electrochemical polymerization of monomers directly underneath the AFM tip. Sub-50 nm poly-3,4-ethylenedioxythiophene lines can be easily created. Such capability to direct-write and pattern polymeric materials with interesting electronic and electrooptical properties at the nanoscale creates a number of opportunities since a large variety of monomers are available.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.