The enthalpically favoured hydration of hydrophobic entities, termed hydrophobic hydration, impacts the phase behaviour of numerous amphiphiles in water. Here, we show experimental evidence that hydrophobic hydration is strongly determined by the mean energetics of the aqueous medium. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. This denotes the energetics of water as a key parameter controlling the phase behaviour of PNiPAM and identifies the excess mixing enthalpy of water/kosmotrope mixtures as a gauge of the kosmotropic effect on hydrophobic assemblies.

Recent advances in designing metamaterials have demonstrated that global mechanical properties of disordered spring networks can be tuned by selectively modifying only a small subset of bonds. Here, using a computationally efficient approach, we extend this idea to tune more general properties of networks. With nearly complete success, we are able to produce a strain between any two target nodes in a network in response to an applied source strain on any other pair of nodes by removing only ∼1% of the bonds. We are also able to control multiple pairs of target nodes, each with a different individual response, from a single source, and to tune multiple independent source/target responses simultaneously into a network. We have fabricated physical networks in macroscopic 2D and 3D systems that exhibit these responses. This work is inspired by the long-range coupled conformational changes that constitute allosteric function in proteins. The fact that allostery is a common means for regulation in biological molecules suggests that it is a relatively easy property to develop through evolution. In analogy, our results show that long-range coupled mechanical responses are similarly easy to achieve in disordered networks. mechanical metamaterials | allostery | tunable response | proteins | disordered networks T he ability to tune the response of mechanical networks has significant applications for designing metamaterials with unique properties. For example, the ratio G/B of the shear modulus G to the bulk modulus B can be tuned by over 16 orders of magnitude by removing only 2% of the bonds in an ideal spring network (1). Such a pruning procedure allows one to create a network that has a Poisson ratio ν anywhere between the auxetic limit (ν = − 1) and the incompressible limit. In another example, the average coordination number of a network controls the width of a failure zone under compression or extension (2). Both these results are specific to tuning the global responses of a material. However, many applications rely on targeting a local response to a local perturbation applied some distance away. For example, allostery in a protein is the process by which a molecule binding locally to one site affects the activity at a second distant site (3). Often this process involves the coupling of conformational changes between the two sites (4). Here we ask whether disordered networks, which generically do not exhibit this behavior, can be tuned to develop a specific allostery-inspired structural response by pruning bonds.We introduce a formalism for calculating how each bond contributes to the mechanical response, anywhere in the network, to an arbitrary applied source strain. The formalism allows us to develop algorithms to control how the strain between two arbitrarily chosen target nodes responds to the strain applied between two arbitrary source nodes. In the simplest case, bonds are removed sequentially until the desired target strain is reached. For almost all of the initial networks studied, only a small fraction of th...

We investigate the co-nonsolvency of poly-N-isopropyl acrylamide (PNiPAM) in different water-alcohol mixtures and show that this phenomenon is due to two distinct solvation contributions governing the phase behavior of PNiPAM in the water-rich and alcohol-rich regime respectively. While hydrophobic hydration is the predominant contribution governing the phase behavior of PNiPAM in the water-rich regime, the mixing contributions governing the phase behavior of classical polymer solutions determine the phase behavior of PNiPAM in the alcohol-rich regime. This is evidenced by distinct scaling relations denoting the energetic state of the aqueous medium as a key parameter for the phase behavior of PNiPAM in the water-rich regime, while the volume fractions of respectively water, alcohol and PNiPAM become relevant parameters in the alcohol-rich regime. Adding alcohol to water decreases the energetics of the aqueous medium, which gradually suppresses hydrophobic hydration, while adding water to alcohol decreases the solvent quality. Consequently, PNiPAM is insoluble in the intermediate range of solvent composition, where neither hydrophobic hydration nor the mixing contributions prevail. This accounts for the co-nonsolvency phenomenon observed for PNiPAM in water-alcohol mixtures.

The invasion of one fluid into another of higher viscosity in a quasi-two dimensional geometry typically produces complex fingering patterns. Because interfacial tension suppresses short-wavelength fluctuations, its elimination by using pairs of miscible fluids would suggest an instability producing highly ramified singular structures. Previous studies focused on wavelength selection at the instability onset and overlooked the striking features appearing more globally. Here we investigate the non-linear growth that occurs after the instability has been fully established. We find a rich variety of patterns that are characterized by the viscosity ratio between the inner and the outer fluid, Z in /Z out , as distinct from the mostunstable wavelength, which determines the onset of the instability. As Z in /Z out increases, a regime dominated by long highly-branched fractal fingers gives way to one dominated by blunt stable structures characteristic of proportionate growth. Simultaneously, a central region of complete outer-fluid displacement grows until it encompasses the entire pattern at Z in /Z out E0.3.

We investigate dynamical heterogeneities in the collective relaxation of a concentrated microgel system, for which the packing fraction can be conveniently varied by changing the temperature. The packing fraction-dependent mechanical properties are characterized by a fluid-solid transition, where the system properties switch from a viscous to an elastic low-frequency behaviour. Approaching this transition from below, we find that the range ξ of spatial correlations in the dynamics increases. Beyond this transition, ξ reaches a maximum, extending over the entire observable system size of approximately 5 mm. Increasing the packing fraction even further leads to a second transition, which is characterized by the development of large zones of lower and higher dynamical activity that are well separated from each other; the range of correlation decreases at this point. This striking non-monotonic dependence of ξ on volume fraction is reminiscent of the behaviour recently observed at the jamming/rigidity transition in granular systems. We identify this second transition as the transition to 'squeezed' states, where the constituents of the system start to exert direct contact forces on each other, such that the dynamics becomes increasingly determined by imbalanced stresses. Evidence of this transition is also found in the frequency dependence of the storage and loss moduli, which become increasingly coupled as direct friction between the particles starts to contribute to the dissipative losses within the system. To our knowledge, our data provide the first observation of a qualitative change in dynamical heterogeneity as the dynamics switches from purely thermally driven to stress driven.

We investigate the phase behaviour of aqueous dispersions of poly-N-isopropyl acrylamide (PNiPAM) microgels above their lower critical solution temperature (LCST) and find that beyond a well-defined concentration the systems exhibit a peculiar behaviour: the microgels assemble into space-spanning gels that shrink in time while maintaining the shape of the container in which they have been formed. Over a wide range of concentrations this shrinking behaviour is independent of PNiPAM concentration, but systematically depends on temperature in a temperature range significantly exceeding the LCST. The overall shrinking characteristics are consistent with those expected for scaffolds made of materials that exhibit thermal contraction. However, for the PNiPAM assemblies contraction is irreversible and can be as large as 90%. Such characteristics disclose complex interactions between fully collapsed PNiPAM and water well beyond the LCST, the origin of which has yet to be elucidated.

To gain insight into the kinetics of colloidal gel evolution at low particle volume fractions φ, we utilize differential dynamic microscopy to investigate particle aggregation, geometric percolation, and the subsequent transition to nonergodic dynamics. We report the emergence of unexpectedly rich multiscale dynamics upon the onset of nonergodicity, which separates the wave vectors q into three different regimes. In the high-q domain, the gel exhibits φ-independent internal vibrations of fractal clusters. The intermediate-q domain is dominated by density fluctuations at the length scale of the clusters, as evidenced by q-independence of the relaxation time τ . In the low-q domain, the scaling of τ as q −3 suggests that the network appears homogeneous. The transitions between these three regimes introduce two characteristic length scales, distinct from the cluster size.Rich rheological behavior of colloidal gels arises from the coexistence of multiple length and time scales that characterize their structure and dynamics. Colloidal gels with low particle volume fractions φ form space-spanning networks of fractal clusters through particle aggregation or phase separation [1][2][3][4][5]. The self-similarity of these fractal aggregates originates in a broad range of length scales that constitute the gels from the size of individual particles to that of clusters. In addition, the kinetic arrest during gelation prevents the systems from ever reaching equilibrium states [6][7][8]. The nonequilibrium gels constantly evolve via structural rearrangements triggered by thermal agitation and residual stresses, even in the absence of external perturbations [9][10][11][12]. The ceaseless change in structure, in turn, leads to a continuous evolution of the dynamics.Understanding the microscopic behavior of colloidal gels, therefore, necessitates both spatially and temporally comprehensive investigation, which poses several challenges to experimentalists. The dynamics of gel networks has been extensively studied with various scattering techniques, such as dynamic light scattering [13][14][15][16][17], diffusing wave spectroscopy [18][19][20][21][22], and X-ray photon correlation spectroscopy [23,24]. Their applications to the characterization of evolving gels, however, have been limited by small ranges of accessible length scales and prolonged data acquisition during which the systems significantly age. This lack of extensivity in experimental characterization has prevented a coherent description of the dynamics of colloidal gels.In this Letter, we trace the entire kinetic pathway, from stable suspensions through aged gels, of colloidal gelation and network evolution over large ranges of length and time scales using differential dynamic microscopy (DDM) [25,26]. The motion of particles and their aggregates initially slows down through two consecutive stages, while the system remains ergodic. As the gel evolves, network fluctuations become greatly suppressed, leading to the onset of nonergodicity. Three dynamically distinct ranges ...

The displacement of a more viscous fluid by a less viscous one in a quasi-two dimensional geometry leads to the formation of complex fingering patterns. This fingering has been characterized by a most unstable wavelength, λc, which depends on the viscosity difference between two immiscible fluids and sets the characteristic width of the fingers. How the finger length grows after the instability occurs is an equally important, but previously overlooked, aspect that characterizes the global features of the patterns. Long after the instability onset, once the fingers are growing in a nearly steady-state regime, there is a stable inner region where the outer fluid is completely displaced. We show that the ratio of the finger length to the radius of this stable region depends only on the viscosity ratio of the fluids and is decoupled from λc.