Structural Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces

Assembly of nanoscale materials from nanoparticle (NP) building blocks relies on our understanding of multiple nanoscale forces acting between NPs. These forces may compete with each other and yield distinct stimuli-responsive self-assembled nanostructures. Here, we report structural transitions between linear chains and globular assemblies of charged, polymer-stabilized gold NPs, which are governed by the competition of repulsive electrostatic forces and attractive poor solvency/hydrophobic forces. We propose a simple quantitative model and show that these transitions can be controlled by the quality of solvent, addition of a salt, and variation of the molecular weight of the polymer ligands

Surface patterning of nanoparticles with polymer patches

Patterning of colloidal particles with chemically or topographically distinct surface domains (patches) has attracted intense research interest1–3. Surface-patterned particles act as colloidal analogues of atoms and molecules4,5, serve as model systems in studies of phase transitions in liquid systems6, behave as ‘colloidal surfactants’7 and function as templates for the synthesis of hybrid particles8. The generation of micrometre- and submicrometre-sized patchy colloids is now efficient9–11, but surface patterning of inorganic colloidal nanoparticles with dimensions of the order of tens of nanometres is uncommon. Such nanoparticles exhibit size- and shape-dependent optical, electronic and magnetic properties, and their assemblies show new collective properties12. At present, nanoparticle patterning is limited to the generation of two-patch nanoparticles13–15, and nanoparticles with surface ripples16 or a ‘raspberry’ surface morphology17. Here we demonstrate nanoparticle surface patterning, which utilizes thermodynamically driven segregation of polymer ligands from a uniform polymer brush into surface-pinned micelles following a change in solvent quality. Patch formation is reversible but can be permanently preserved using a photocrosslinking step. The methodology offers the ability to control the dimensions of patches, their spatial distribution and the number of patches per nanoparticle, in agreement with a theoretical model. The versatility of the strategy is demonstrated by patterning nanoparticles with different dimensions, shapes and compositions, tethered with various types of polymers and subjected to different external stimuli. These patchy nanocolloids have potential applications in fundamental research, the self-assembly of nanomaterials, diagnostics, sensing and colloidal stabilization

On Second-Order Monadic Monoidal and Groupoidal Quantifiers

We study logics defined in terms of second-order monadic monoidal and groupoidal quantifiers. These are generalized quantifiers defined by monoid and groupoid word-problems, equivalently, by regular and context-free languages. We give a computational classification of the expressive power of these logics over strings with varying built-in predicates. In particular, we show that ATIME(n) can be logically characterized in terms of second-order monadic monoidal quantifiers

The new scalar politics of evaluation: An emerging governance role for evaluation

In this article we analyze how roles for evaluation are described and argued for in key texts produced and/or promoted by three influential international networks: the High Level Forum on Aid Effectiveness; the Organisation for Economic Cooperation and Development Assistance Committee’s Network on Development Evaluation; and the Network of Networks for Impact Evaluation. We contend that these complex multilateral networks are working supranationally through soft power to promote: common standards of evaluation practice; a dominant model of evaluation (impact evaluation); and new evaluation roles, relationships and practices for the field of development. Moreover, we argue that this emerging complex multilateral agenda for evaluation may position evaluation and evaluators within a global governance strategy allowing greater influence to international development organizations. We conclude with a discussion of the implications of the analysis for evaluators working in the field of international development

Pine cone scale-inspired motile origami

Stimuli-sensitive hydrogels have received attention because of their potential applications in various fields. Stimuli-directed motion offers many practical applications, such as in drug delivery systems and actuators. Directed motion of asymmetric hydrogels has long been designed; however, few studies have investigated the motion control of symmetric hydrogels. We designed a pine cone scale-inspired movable temperature-sensitive symmetric hydrogel that contains Fe3O4. Alignment of Fe3O4 along the magnetic force is key in motion control in which Fe3O4 acts like fibers in a pine cone scale. Although a homogeneous temperature-sensitive hydrogel cannot respond to a temperature gradient, the Fe3O4-containing hydrogel demonstrates considerable bending motion. Varying degrees and directions of motion are easily facilitated by controlling the amount and alignment angle of the Fe3O4. The shape of the hydrogel layer also influences the morphological structure. This study introduced facile and low-cost methods to control various bending motions. These results can be applied to many fields of engineering, including industrial engineering.111Ysciescopu