Binhua Lin

Stress and Fold Localization in Thin Elastic Membranes

Thin elastic membranes supported on a much softer elastic solid or a fluid deviate from their flat geometries upon compression. We demonstrate that periodic wrinkling is only one possible solution for such strained membranes. Folds, which involve highly localized curvature, appear whenever the membrane is compressed beyond a third of its initial wrinkle wavelength. Eventually the surface transforms into a symmetry-broken state with flat regions of membrane coexisting with locally folded points, reminiscent of a crumpled, unsupported membrane. We provide general scaling laws for the wrinkled and folded states and proved the transition with numerical and experimental supported membranes. Our work provides insight into the interfacial stability of such diverse systems as biological membranes such as lung surfactant and nanoparticle thin films.

Surface Crystallization in a Liquid AuSi Alloy

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloys melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.

Structural transitions in a monolayer of fluorinated amphiphile molecules

We present results of an extensive x‐ray diffraction study of a monolayer of C10F21CH2COOH spread on water (pH2) at 19.1 °C. Lever‐rule analysis of the in‐plane scattering is used to show that there is a coexistence region between ordered condensed islands and a dilute disordered phase. The coexistence region is found to be bounded by molecular areas of 29 A2 and 2000±600 A2, in agreement with the pressure‐area isotherm. The molecular tilt of the ordered phase remains unchanged from closest packing (near collapse of the monolayer) throughout the coexistence region, and has a value of 2±3 degrees with respect to the normal to the liquid surface. These results are contrasted with those for hydrocarbon monolayers in which the onset of order in the coexistence region is close to the condensed phase boundary, and the ordered phase is compressible with a continuously variable tilt angle ranging from 30 to 0 deg at closest packing. The differences are attributed to the enhanced chain stiffness of the fluorinated...

From random walk to single-file diffusion.

We report an experimental study of diffusion in a quasi-one-dimensional (q1D) colloid suspension which behaves like a Tonks gas. The mean squared displacement as a function of time is described well with an ansatz encompassing a time regime that is both shorter and longer than the mean time between collisions. The ansatz asserts that the inverse mean squared displacement is the sum of the inverse mean squared displacement for short time normal diffusion (random walk) and the inverse mean squared displacement for asymptotic single-file diffusion (SFD). The dependence of the 1D mobility in the SFD on the concentration of the colloids agrees quantitatively with that derived for a hard rod model, which confirms for the first time the validity of the hard rod SFD theory. We also show that a recent SFD theory by Kollmann leads to the hard rod SFD theory for a Tonks gas.

Screened Hydrodynamic Interaction in a Narrow Channel

We study experimentally and theoretically the hydrodynamic coupling between Brownian colloidal particles diffusing along a linear channel. The quasi-one-dimensional confinement, unlike other constrained geometries, leads to a sharply screened interaction. Consequently, particles move in concert only when their mutual distance is smaller than the channel width, and two-body interactions remain dominant up to high particle densities. The coupling in a cylindrical channel is predicted to reverse sign at a certain distance, yet this unusual effect is too small to be currently detectable.

Dynamical heterogeneity in a dense quasi-two-dimensional colloidal liquid

This paper reports the results of experimental studies of the dynamics of particles in a dense quasi-two-dimensional colloidal liquid. We find that at high density, near close packing but still in the liquid phase, the spatial configurations of the particles in the colloidal liquid consist of small ordered domains separated by disordered boundaries. There are frequent exchanges of particles between the ordered and disordered domains, so the lifetime of a particular ordered domain is short and the state of the system is ergodic. The motion of a particle in an ordered domain is constrained but fully two dimensional. The motion of a particle in a disordered boundary has considerable one-dimensional file-server character. By virtue of exchanges of particles between the ordered and disordered domains, the time dependence of the particle displacement has mixed character. We find that the particle dynamics in the dense quasi-two-dimensional colloidal liquid can be characterized with three simultaneous competing ...

Tuning ion correlations at an electrified soft interface

Ion distributions play a central role in various settings—from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength of ion–ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameter-free density functional theory that includes ion–ion correlations and ion–solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.

Atomic-Scale Surface Demixing in a Eutectic Liquid BiSn Alloy

Resonant x-ray reflectivity of the surface of the liquid phase of the Bi(43)Sn(57) eutectic alloy reveals atomic-scale demixing extending over three near-surface atomic layers. Because of the absence of an underlying atomic lattice which typically defines adsorption in crystalline alloys, studies of adsorption in liquid alloys provide unique insight on interatomic interactions at the surface. The observed composition modulation could be accounted for quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of the single-layer Gibbs model, revealing a near-surface domination of the attractive Bi-Sn interaction over the entropy.

Anomalous layering at the liquid sn surface

X-ray reflectivity measurements on the free surface of liquid Sn are presented. They exhibit the high-angle peak, indicative of surface-induced layering, also found for other pure liquid metals (Hg, Ga, and In). However, a low-angle shoulder, not hitherto observed for any pure liquid metal, is also found, indicating the presence of a high-density surface layer. Fluorescence and resonant reflectivity measurements rule out the assignment of this layer to surface segregation of impurities. The reflectivity is modeled well by a 10% contraction of the spacing between the first and second atomic surface layers, relative to that of subsequent layers. Possible reasons for this are discussed.

Molecular Structure of Interfacial Human Meibum Films

Meibum is the primary component of the tear film lipid layer. Thought to play a role in tear film stabilization, understanding the physical properties of meibum and how they change with disease will be valuable in identifying dry eye treatment targets. Grazing incidence X-ray diffraction and X-ray reflectivity were applied to meibum films at an air-water interface to identify molecular organization. At room temperature, interfacial meibum films formed two coexisting scattering phases with rectangular lattices and next-nearest neighbor tilts, similar to the Ov phase previously identified in fatty acids. The intensity of the diffraction peaks increased with compression, although the lattice spacing and molecular tilt angle remained constant. Reflectivity measurements at surface pressures of 18 mN/m and above revealed multilayers with d-spacings of 50 Å, suggesting that vertical organization rather than lateral was predominantly affected by meibum-film compression.