Adaptive architecture and mechanoresponse of epithelial cells on a torus.

Curvature is a geometric feature widely observed in the epithelia and critical to the performance of fundamental biological functions. Understanding curvature-related biophysical phenomena remains challenging partly owing to the difficulty of quantitatively tuning and measuring curvatures of interfacing individual cells. In this study, we prepared confluent wild-type Madin-Darby canine kidney cells on a torus structure presenting positive, zero, and negative Gaussian curvatures with a tubule diameter of 2-7 cells and quantified the mechanobiological characteristics of individual cells. Cells on the torus surface exhibited topological sensing ability both as an individual cell and collective cell organization. Both cell bodies and nuclei, adapted on the torus, exhibited local Gaussian curvature-dependent preferential orientation. The cells on the torus demonstrated significant adjustment in the nuclear area and exhibited asymmetric nuclear position depending on the local Gaussian curvature. Moreover, cells on top of the torus, where local Gaussian curvature is near zero, exhibited more sensitive morphological adaptations than the nuclei depending on the Gaussian curvature gradient. Furthermore, the spatial heterogeneity of intermediate filament proteins related to mechanoresponsive expression of the cell body and nucleus, vimentin, keratin and lamin A, revealed local Gaussian curvature as a key factor of cellular adaptation on curved surfaces.

Rotating crystals of magnetic Janus colloids.

Monodisperse magnetic colloids are found to self-assemble into unusual crystals in the presence of rotating magnetic fields. First, we confirm a predicted phase transition (S. Jäger and S. H. L. Klapp, Soft Matter, 2011, 7, 6606-6616), directly coupled to the dynamic transition of single particle motion, from a disordered state to a hexagonal crystal. Next, going beyond what had been predicted, we report how hydrodynamic coupling produces shear melting, dislocations, and periodically mobile domain boundaries. These uniform magnetic colloids, whose structures are modulated in situ using the protocols described here, demonstrate a strategy of stimulus-response in the colloid domain with potential applications.

Nanotribology, standard friction, and bulk rheology properties compared for a Brij microemulsion.

A microemulsion consisting of Brij 96, glycerol (co-surfactant), oil, and water was compared as concerns deformations in a surface forces apparatus whose surface where rendered hydrophobic by coating with a monolayer of condensed OTE (octadecyltriethoxysilane), as concerns tribology of the conventional kind during sliding between hydrophobic PDMS surfaces, and as concerns bulk rheology. In the bulk, light scattering characterization showed swollen spherical micelles with a 13 nm diameter. When squeezed to form thinner films than this, the effective viscosity measured rose by orders of magnitude. It appears that thin films in the range of thickness 13 to 7 nm are comprised of deformed micelles and that confinement to thinner films expels micelles with concomitant even more drastic structural deformation of the remaining micelles, until the thinnest films retain only adsorbed surfactant. Tentatively, this may explain why the friction response then became similar to that of surfactant itself [M. Graca, J.H.H. Bongaerts, J.R. Stokes, S. Granick, J. Colloid Interface Sci. 315 (2007) 662]. These measurements are considered to be the first comparison of microemulsion rheology in the bulk and in nanometer-thick films.

Confinement-induced differences between dielectric normal modes and segmental modes of an ion-conducting polymer.

Dielectric measurement in the range 0.1 Hz to 1 MHz were used to study the motions of polymers and ions in an ion-conducting polymer, polypropylene oxide containing small quantities (on the order of 1%) of lithium ions (LiClO(4)), confined as a sandwich of uniform thickness between parallel insulating mica surfaces. In the dielectric loss spectrum, we observed three peaks; they originated from the normal mode of the polymer, segmental mode of the polymer, and ion motions. With decreasing film thickness, the peak frequencies corresponding to the normal mode and ion motion shifted to lower frequencies, indicating retardation due to confinement above 30 nm. This was accompanied by diminished intensity of the dielectric normal-mode relaxation, suggesting that confinement diminished the fluctuations of the end-to-end vector of the chain dipole in the direction between the confining surfaces. On the contrary, the segmental mode was not affected at that thickness. Finally, significant retardation of the segmental mode was observed only for the thinnest film (14 nm). The different dynamical modes of the polymer (segmental and slowest normal modes) respond with different thickness and temperature dependence to confinement.

Viscosity of interfacial water.

The effective shear viscosity and frequency-dependent dynamic oscillatory shear spectra of water containing monovalent or divalent ions (ionic strength 25 mM), confined between mica crystals at 1-2 water molecules thickness, oscillated with twist angle with the period expected for the pseudohexagonal surface lattice. The effective viscosity varied by orders of magnitude as the twist angle was changed. Confinement appeared to imprint lateral spatial correlation on the ultrathin liquid, the more so the better the confining lattices were aligned, but the oft-proposed "ice structure" was not observed dynamically.

Rate-dependent slip of Newtonian liquid at smooth surfaces.

Newtonian fluids were placed between molecularly smooth surfaces whose spacing was vibrated at spacings where the fluid responded as a continuum. Hydrodynamic forces agreed with predictions from the no-slip boundary condition only provided that flow rate (peak velocity normalized by spacing) was low, but implied partial slip when it exceeded a critical level, different in different systems, correlated with contact angle (surface wettability). With increasing flow rate and partially wetted surfaces, hydrodynamic forces became up to 2-4 orders of magnitude less than expected by assuming the no-slip boundary condition that is commonly stated in textbooks.

The consequences of drug use/abuse for vocational career: a longitudinal study of a male urban African-American sample.

This report is from a longitudinal study of a community sample of African-American males (N = 197) on the relationship of the degree of earlier substance use/abuse up to average age 24, to vocational performance (employment and occupational level) 2 1/2 years later (at average age 26 1/2). The statistical analyses included numerous control variables developed from prospective data of the National Collaborative Perinatal Project, on the subjects and on their families from the subjects' birth to age 7, and from their school behavior and academic performance up to age 16, which may have influenced their vocational-occupational behavior during early adulthood. It was found that greater earlier marijuana use and greater earlier alcohol use predicted, to a significant degree, poorer occupational performance.

The relationships of substance abuse to illegal and violent behavior, in a community sample of young adult African American men and women (gender differences).

In a longitudinal study of an African American young adult community sample (N = 380), prospective data on lifetime substance use/abuse from childhood up to age 24 were used as control variables in analyses to predict illegal and violent behavior during the ensuing 2 1/2-year period. Frequent earlier use of drugs predicted subsequent violent behavior for both men and women. Frequency of earlier use of alcohol predicted subsequent violent behavior for men but not for women. A weaker relationship was found between degree of psychopathology and degree of engaging in either illegal or violent behavior than between degree of psychopathology and degree of substance use/abuse. Comorbidity (the combination of earlier use/abuse of drugs with earlier psychopathology) was a stronger predictor, for women than for men, of later illegal and violent behavior.

Gender differences in early life risk factors for substance use/abuse: a study of an African-American sample.

Gender differences in risk and protective factors for substance use/abuse in early adulthood were studied. Comprehensive systematic data on African-American males (N = 318) and females (N = 322), from birth to 7 years of age, were available from the National Collaborative Perinatal Study. These subjects were retrieved for assessment at average age 24. There are more differences between males and females than there are similarities in regard to the early childhood variables that predict substance use in early adulthood. However, high activity and intensity of response during infancy (measured at 8 months of age) was found to predict later substance use for both males and females. This type of behavior is considered by use to be a trait of temperament and to suggest the possibility of a genetic predisposition. More risk factors were found for female than for males. The risk factors for females were primarily of two types: 1) Related to experiences with mother and with the family environment; and 2) Poor levels of intellectual functioning and academic performance, and abnormal mental status.

From static to kinetic friction in confined liquid films.

The transition from rest to sliding contact of atomically smooth solids separated by molecularly thin liquid films was studied. The films could be deformed nearly reversibly to a large fraction of the film thickness. The modulus of elasticity and yield stress were low, considerably less than for a molecular crystal or glass in the bulk. The transition to dissipative sliding was typically (but not always) discontinuous. The dissipative stress was then nearly velocity-independent. The similar response of monolayers strongly attached to the solid surfaces, presenting a well-defined interface for sliding, suggests that the physical mechanism of sliding may involve wall slip.

A simple kinetic model of polymer adsorption and desorption.

A model of the desorption and adsorption of a polymer layer at a planar surface indicates a transition from exponential kinetics at high temperatures to nonexponential kinetics (stretched exponential with index one-half) at lower temperatures where these processes are diffusion-limited. Measurements of polystyrene desorption through polyisoprene overlayers show this predicted transition. Corroborative results are obtained for polystyrene desorption through polymethylmethacrylate overlayers. This identification of two distinct kinetic regimes suggests a unifying perspective from which to analyze polymer and biopolymer mobility at surfaces.

Viscoelastic dynamics of confined polymer melts.

The frequency-dependent shear response of an ultrathin polymer melt (polyphenylmethylsiloxane) confined between adsorbing surfaces (parallel plates of mica) is described. The sinusoidal deformations were sufficiently small to give linear response, implying that measurement did not perturb the film structure. A remarkable transition was observed with decreasing thickness. When the film thickness was less than five to six times the unperturbed radius of gyration, there emerged a strong rubber-like elasticity that was not characteristic of the bulk samples. This result indicates enhanced entanglement interactions in thin polymer films and offers a mechanism to explain the slow mobility of polymers at surfaces.

New mechanism of nonequilibrium polymer adsorption.

Nonequilibrium states of surface composition can be extremely long-lived when polymer chains adsorb competitively. In a model system (polymethylmethacrylate adsorbed from CCl(4) onto oxidized silicon previously saturated with polystyrene), it is shown that a weakly adsorbing polymer was sterically pinned to a surface by a more strongly adsorbing polymer. The dynamical evolution of the surface composition was strongly nonexponential in time and non-Arrhenius in temperature; the phenomenology is analogous to bulk glasses. This interpretation offers a new mechanism to explain why weakly adsorbing chains may bind to surfaces, as well as a direction in which to look for a method to release them.