Liquid-like Response Research Articles

Structures, solvation forces and shear of molecular films in a rough nano-confinement

Investigations of surface roughness effects on the structure, dynamics and rheology of a molecular fluid (hexadecane) confined between solid (gold) surfaces, through the use of large-scale molecular dynamics simulations, reveal a remarkable sensitivity to the confining surface morphology. A most significant reduction of the ordering propensity is found in films confined by stationary rough surfaces with a consequent strong suppression of solvation forces and the development of liquid-like dynamic and response characteristics. When the rough-surface boundaries are set in motion at a high shear rate, the interfacial layers of the film stick to the adjacent solid boundaries, resulting in partial slip inside the film with the development of shear stress in the viscous molecular fluid, unlike the case of atomically flat crystalline boundaries where slip of the confined film at the boundaries is accompanied by vanishingly small shear stress in the film. These results are discussed in the context of the effect of roughness on the boundary conditions used in modeling fluid flow past surfaces, and they suggest that morphological patterning of surfaces could provide ways for controlled modifications of frictional processes in thin-film lubricated nanotribological systems.

Molecular order versus vitrification in high-sugar blends of gelatin and kappa-carrageenan.

The understanding of synthetic polymer viscoelasticity was applied to the small deformation properties of high-sugar gelatin and kappa-carrageenan mixtures. The glass transition zone in sugar/gelatin mixtures exhibited a dominant liquid-like response, which was followed by the method of reduced variables. The glass transition temperature predicted by the WLF/free volume approach coincided with the crossover of storage and loss modulus at the onset of the glassy state. The viscoelastic spectrum was resolved into a basic function of temperature and a basic function of time, thus rationalizing their effect on the vitrification of the mixture. Vitrification was only one in a plethora of viscoelastic properties. Manipulation of the composition of the mixture generated a continuous gelatin matrix or a viscous sugar phase suspending a dispersion of ice crystals. Besides the small-molecule crystallinity, vitrification can be halted by ordered macromolecular helices observed in the kappa-carrageenan/potassium system. Thus, the solid-like component of the sugar/kappa-carrageenan network remained prevalent over the predicted frequency window. The WLF equation was unable to follow the progress of viscoelastic functions, which were better described by an energetic barrier of rotation from one state to another.

Orientation in a Fatty Acid Monolayer: Effect of Flow Type

The two-dimensional fluid dynamics of the different phases of a fatty acid monolayer (docosanoic acid) were examined. Using Brewster angle microscopy, we studied the polydomain structure of two liquid condensed phases (the L 2 and L' 2 phases) and the solid, S, phase in situ during the application of extensional flow and simple shear flow. We found that only the L2 phase deformed nearly reversibly with a liquid-like response. Nonsymmetric domain deformations were, however, found for that phase at the lowest rate of strain studied with a four-roll mill. At higher strain rates, plots of the evolution of strain against time collapsed onto a single curve so that the rate of strain was independent of roller speed. Furthermore, a critical strain y c exists above which the rate of strain shows a stepwise increase despite the constant velocity of the rollers. The two other phases, the L' 2 and S phases, experienced flow-induced reorientation of the lattice onto which the molecules are arranged. The reorientation process was accompanied by the appearance of shear bands in the monolayers at ±45° to the extension axis of both types of flow. The shear bands observed in the L' 2 phase were modeled as a plastic flow accompanying the molecular tilt reorientation developed within elastic regions of the monolayer. This model describes the time evolution of the bandwidth quite well and provides strong evidence of the existence of an additional phase within the conventional L' 2 phase region. In simple shear flow, the velocity profile for the L 2 phase across the gap of the shearing cell showed a nonlinear distribution of shear rates, which were highest at the center of the gap. Flow-induced breakup of domains was observed in the L' 2 phase subject to simple shear.

Phase coherence along the c-axis in Bi 2Sr 2CaCu 2O 8+ y with columnar defects probed by Josephson plasma resonance

We studied the c-axis phase coherence in Bi 2Sr 2CaCu 2O 8+ y with columnar defects using the Josephson plasma resonance as a probe. At temperatures close to T c, the Josephson plasma resonance field ( H p) shows a characteristic temperature dependence of decoupled pancake vortices similar to the pristine Bi 2Sr 2CaCu 2O 8+ y . The drastic increase in H p occurs at a frequency independent characteristic field even in the vortex liquid phase. We interpret this change as a manifestation of recoupling of decoupled vortices by the columnar defects. The increase in H p becomes larger with increasing the density of columnar defects. The temperature dependence of H p at lower temperatures but above the irreversibility line can be understood by considering two kinds of vortices, some trapped in the columnar defects and others located between them giving a decoupled liquid-like response.

Flow-Induced Molecular Orientation of a Langmuir Film

The two-dimensional fluid dynamics of the different phases of a fatty acid monolayer (docosanoic acid) were examined. Brewster angle microscopy was used to investigate the polydomain structure of two "liquid condensed" phases (the L 2 and L' 2 phases) and the "solid" (S) phase in situ during an extensional flow. Only the L 2 phase deformed affinely with a liquid-like response. The two other phases experienced flow-induced reorientation of the lattice onto which the molecules were arranged. The reorientation process was accompanied by the appearance of shear bands in the monolayers at an angle of ±45 degrees to the extension axis of the flow.

Ac response of the flux-line liquid in the high-Tc superconductors.

We use a hydrodynamics theory to discuss the response of a viscous flux-line liquid to an ac perturbation applied at the surface of the sample. The theory incorporates viscoelastic effects and describes the crossover between liquidlike and solidlike response of the vortex array as the frequency of the perturbation increases. A large viscosity from flux-line interactions and entanglement leads to viscous screening of surface fields. As a result, two frequency-dependent length scales are needed to describe the penetration of an ac field. For large viscosities the imaginary part of the ac permeability can exhibit, in addition to the well-known peak associated with flux diffusion across the sample, a low-frequency peak corresponding to the transition from solidlike to liquidlike behavior.

Shear rheology in a confined geometry: polysiloxane melts

The dynamic mechanical shear response was measured of sharp fractions of molten siloxane oligomers, PDMS ( poly(dimethylsiloxane)) and PPMS ( poly(phenylmethylsiloxane)), confined between single crystals of muscovite mica, at film thicknesses e100 A and a temperature of 23 C. Five conclusions emerge. (1) A liquidlike mechanical response (in which the apparent dynamic viscosity was significantly enhanced over that of the bulk liquid) was clearly distinguished from a yield stress response (in which sliding over the experimental time scale occurred only after a critical yield stress was exceeded). These same features were observed previously for ultrathin films of smaller nonpolar molecules, and, despite quantitative differences in the present systems, the observation appears to be general. (2) The precise film thickness at the onset of the yield stress response, observed at film thicknesses C30-50 A, did not depend on the molecular weight of the PDMS fractions but did depend markedly on details of the history of the experiment. (3) The yield stress increased with measurement time without a discernible change in separation. The times for the yield stress to reach a plateau increased with molecular weight and ranged from approximately 9 min (PDMS, Mn = 890) to approximately 400 min (PDMS, Mn = 6330) and approximately 650 min (PPMS, M, = 2240). (4) Enhanced viscous response was observed at larger film thickness than for liquids of smaller molecules. The distance dependence of the apparent dynamic viscwity at 0.875 Hz was quantified for one sample (PDMS, M, = 1670) by measuring the phase shift and amplitude attenuation in sinusoidal oscillation. The apparent dynamic viscosity appeared to diverge with diminishing film thickness. (5) After discussing how the act of shear may affect the structure of the liquid, we conclude that the yield stress rheological response may reflect a metastable, history-dependent state, in which relaxations of trapped chains have become slower than the experimental time scale of minutes to hours.