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Emulsions often act as carriers for water-insoluble solutes that are delivered to a specific target. The molecular transport of solutes in emulsions can be facilitated by surfactants and is often limited by diffusion through the continuous phase. We here investigate this transport on a molecular scale by using a lipophilic molecular rotor as a proxy for solutes. Using fluorescence lifetime microscopy we track the transport of these molecules from the continuous phase towards the dispersed phase in polydisperse oil-in-water emulsions. We show that this transport comprises two timescales, which vary significantly with droplet size and surfactant concentration, and, depending on the type of surfactant used, can be limited either by transport across the oil-water interface, or by diffusion through the continuous phase. By studying the time-resolved fluorescence of the fluorophore, accompanied by molecular dynamics simulations, we demonstrate how the rate of transport observed on a macroscopic scale can be explained in terms of the local environment that the probe molecules are exposed to.

We show that the surface of ice is self-healing: micrometer deep scratches in the ice surface spontaneously disappear by relaxation on a time scale of roughly an hour. Following the dynamics and comparing it to different mass transfer mechanisms, we find that sublimation from and condensation onto the ice surface is the dominant self-healing mechanism. The self-healing kinetics shows a strong temperature dependence, following an Arrhenius behavior with an activation energy of $\Delta E = 58.6 \pm 4.6$ kJ/mole, agreeing with the proposed sublimation mechanism, and at odds with surface diffusion or fluid flow or evaporation-condensation from a quasi-liquid layer., Comment: 9 pages, 5 figures

We investigate the process of the slow unrolling of a roll of typical pressure-sensitive adhesive, Scotch tape, under its own weight. Probing the peeling velocities down to nm/s resolution, which is three orders of magnitudes lower than earlier measurements, we find that the speed is still non-zero. Moreover, the velocity is correlated to the relative humidity. A humidity increase leads to water uptake, making the adhesive weaker and easier to peel. At very low humidity, the adhesive becomes so stiff that it mainly responds elastically, leading to a peeling process akin to interfacial fracture. We provide a quantitative understanding of the peeling velocity in the two regimes., Comment: 6 pages, 4 figures and Supplemental material

Deposits from evaporating drops have shown to take a variety of shapes, depending on the physicochemical properties of both solute and solvent. Classically, the evaporation of drops of colloidal suspensions leads to the so-called coffee ring effect, caused by radially outward flows. Here we investigate deposits from evaporating drops containing living motile microalgae (Chlamydomonas reinhardtii), which are capable of resisting these flows. We show that utilizing their light-sensitivity allows to control the final pattern: adjusting the wavelength and incident angle of the light source enables to force the formation, completely suppress and even direct the spatial structure of algal coffee rings.

Of all complex fluids, it is probably the rheology of polymers we understand best. In-depth insight into the entanglement and reptation of individual polymers allows us to predict for instance the shear-thinning rheology and the behaviour in virtually any flow situation of practical importance. The situation is markedly different when we move from passive to active polymers where the coupling of filament activity, hydrodynamic interactions, and conformations open the way to a plethora of novel structural and dynamical features. Here we experimentally study the rheology of long, slender and entangled living worms (Tubifex tubifex) and propose this system as a new type of active polymer. Its level of activity can be controlled by changing the temperature or by adding small amounts of alcohol to make the worms temporarily inactive. We find that (i) shear thinning is reduced by activity, (ii) the characteristic shear rate for the onset of shear-thinning is given by the time scale of the activity, and (iii) the low shear viscosity as a function of concentration shows a very different scaling from that of regular polymers. Our study paves the way towards a new research field of `living polymers'., Comment: 3,5 pages and 3 figures

The striking anomalies in physical properties of supercooled water that were discovered in the 1960-70s, remain incompletely understood and so provide both a source of controversy amongst theoreticians, and a stimulus to experimentalists and simulators to find new ways of penetrating the "crystallization curtain" that effectively shields the problem from solution. Recently a new door on the problem was opened by showing that, in ideal solutions, made using ionic liquid solutes, water anomalies are not destroyed as earlier found for common salt and most molecular solutes, but instead are enhanced to the point of precipitating an apparently first order liquid-liquid transition. The evidence was a spike in apparent heat capacity during cooling that could be fully reversed during reheating before any sign of ice crystallization appeared. Here, we use decoupled-oscillator infrared spectroscopy to define the structural character of this phenomenon using similar down and upscan rates as in the calorimetric study. Thin-film samples also permit slow scans (1 K/min) in which the transition has a width of less than 1 K, and is fully reversible. The OH spectrum changes discontinuously at the phase-transition temperature, indicating a discrete change in hydrogen-bond structure. The spectral changes show that the low-temperature liquid is more strongly hydrogen bonded and less disordered as compared to the high-temperature liquid. The spectrum of the low-temperature liquid is essentially that seen in low-density amorphous water. This similarity suggests that the liquid-liquid transition observed here also exists in neat undercooled water, providing a unified explanation for many of its anomalies., Comment: 8 manuscript pages (including 5 figures) + 3 pages of extended data

Salt bridges are known to be important for the stability of protein conformation, but up to now it has been difficult to study their geometry in solution. Here we characterize the spatial structure of a model salt bridge between guanidinium (Gdm+) and acetate (Ac-) using two-dimensional vibrational (2D-IR) spectroscopy. We find that as a result of salt bridging the infrared response of Gdm+ and Ac- change significantly, and in the 2D-IR spectrum, salt bridging of the molecules appears as cross peaks. From the 2D-IR spectrum we determine the relative orientation of the transition-dipole moments of the vibrational modes involved in the salt bridge, as well as the coupling between them. In this manner we reconstruct the geometry of the solvated salt bridge.

We have investigated the simultaneous absorption of near-infrared photons by pairs of neighboring molecules in liquid methanol. Simultaneous absorption by two OH-stretching modes is found to occur at an energy higher than the sum of the two absorbing modes. This frequency shift arises from interaction between the modes, and its value has been used to determine the average coupling between neighboring methanol molecules. We find a rms coupling strength of 46+/-1 cm-1, much larger than can be explained from transition-dipole coupling, suggesting that hydrogen-bond mediated interactions between neighboring molecules play an important role in liquid methanol. The most important aspect of simultaneous vibrational absorption is that it allows for a quantitative investigation of hydrogen-bond cooperativity. We derive the extent to which the hydrogen-bond strengths of neighboring molecules are correlated by comparing the line shape of the absorption band caused by simultaneous absorption with that of the fundamental transition. Surprisingly, neighboring hydrogen bonds in methanol are found to be strongly correlated, and from the data we obtain a hydrogen-bond correlation coefficient of 0.69+/-0.12., Comment: 4 pages, 5 figures, revtex4; submitted to PRL