Dynamical heterogeneity in a highly supercooled liquid under a sheared situation

In the present study, we performed molecular dynamics simulations and investigated dynamical heterogeneity in a supercooled liquid under a steady shear flow. Dynamical heterogeneity can be characterized by three quantities: the correlation length ξ(4)(t), the intensity χ(4)(t), and the lifetime τ(hetero)(t). We quantified all three quantities by means of the correlation functions of the particle dynamics, i.e., the four-point correlation functions, which are extended to the sheared condition. Here, to define the local dynamics, we used two time intervals t = τ(α) and τ(ngp); τ(α) is the α-relaxation time, and τ(ngp) is the time at which the non-Gaussian parameter of the Van Hove self-correlation function is maximized. We discovered that all three quantities (ξ(4)(t), χ(4)(t), and τ(hetero)(t)) decrease as the shear rate γ of the steady shear flow increases. For the time interval t = τ(α), the scalings ξ(4)(τ(α))~γ(-0.08), χ(4)(τ(α))~γ(-0.26), and τ(hetero)(τ(α))~γ(-0.88) were obtained. The steady shear flow suppresses the heterogeneous structure as well as the lifetime of the dynamical heterogeneity. In addition, we demonstrated that all three quantities in the sheared non-equilibrium state can be mapped onto those in the equilibrium state through the α-relaxation time τ(α). This finding means that the same relation between τ(α) and three quantities holds in both the equilibrium state and the sheared non-equilibrium state and therefore proposes that the dynamical heterogeneity can play a similar role in the drastic change of τ(α) due to not only the temperature but also the shear rate.     

Surfactant-assisted spreading of a liquid drop on a smooth solid surface

Axisymmetric spreading of a liquid drop covered with an insoluble surfactant monolayer on a smooth solid substrate is numerically investigated. As the drop spreads, the adsorbed surfactant molecules are constantly redistributed along the air-liquid interface by convection and diffusion, leading to nonuniformities in surface tension along the interface. The resulting Marangoni stresses affect the spreading rate by altering the surface flow and the drop shape. In addition, surfactant accumulation in the vicinity of the moving contact line affects the spreading rate by altering the balance of line forces. Two different models for the constitutive relation at the moving contact line are used, in conjunction with a surface equation of state based on the Frumkin adsorption framework, to probe the surfactant influence. The coupled evolution equations for the drop shape and monolayer concentration profile are integrated using a pseudospectral method to determine the rate of surfactant-assisted spreading over a wide range of the dimensionless parameters governing the spreading process. The insoluble monolayer enhances spreading through two mechanisms; a reduction in the equilibrium contact angle, and an increase in the magnitude of the radial pressure gradient within the drop due to the formation of positive surface curvature near the moving contact line. Both mechanisms are driven by the accumulation of surfactant at the contact line due to surface convection. Although the Marangoni stresses induced at the air-liquid interface reduce the rate of spreading during the initial stages of spreading, their retarding effect is overwhelmed by the favorable effects of the aforementioned mechanisms to lead to an overall enhancement in the rate of spreading in most cases. The spreading rate is found to be higher for bulkier surfactants with stronger repulsive interactions. With the exception of monolayers with strong cohesive interactions which tend to retard the spreading process, the overall effect of an insoluble monolayer is to increase the rate of drop spreading. Simulation results for small Bond numbers indicate the existence of a power-law region for the time-dependence of the basal radius of the drop, consistent with experimental measurements.     

Stress relaxation behaviour of dipalmitoyl phosphatidylcholine monolayers spread on the surface of a pendant drop

Dipalmitoyl phosphatidylcholine (DPPC) monolayers were characterised by surface pressure/area isotherms (π/A) and surface dilational rheological parameters at temperatures 20–40°C. The methods used were the Langmuir trough and the pendant drop micro-film balance. The latter allows accurate measurements at higher temperatures and transient drop deformation. Stable DPPC monolayers were found only for low surface pressures, π<15 mN m⁻¹. At higher monolayer compression π decreases over a long time, mainly caused by molecular rearrangement processes in the monolayer starting in the coexisting region. At π>25 mN m⁻¹ and 20°C relaxation experiments give evident of rupturing, brittle monolayer structures. At higher temperatures the monolayers became more fluid-like. π/A-isotherms determined by using both methods principally agree with each other, but show also remarkable differences, which cannot be explained so far satisfactory. Transient drop relaxation experiments were analysed for the short time range (600 s). At 20°C the dilational modulus (_r) and the surface dilational viscosity (ξ_r) passes a stationary maximum at 0.54 nm² molecule⁻¹ and increase strongly at higher surface coverage, thus indicating crystalline monolayer structure. Increasing temperature from 20 to 30°C causes a rapid decrease of _r and ξ_r and a shift of the stationary maximum to lower surface coverage. No evidence for crystalline structure is found. Further increase of temperature causes _r and ξ_r increase again. This increase is caused by a rising relaxation time, while the elasticity does not change in the same manner. Such intermediate decrease of _r and ξ_r in the range 30–40°C appears to be unusual and can be interpreted as a consequence of strong DPPC interactions and strongly pronounced retardation of monolayer deformation. The study is discussed in connection to the physiology of breathing. For pulmonary surfactants the observed behaviour seems to be understandable. It is however interesting that such complex behaviour is observed for monolayers consisting of DPPC only.     

Shape relaxation of an elongated viscous drop

The shape relaxation of a distorted viscous drop suspended in a quiescent immiscible liquid is analyzed in the creeping flow limit. The shape of the drop is axisymmetric, but otherwise arbitrary. The relaxation process is assumed to be driven by a constant interfacial tension and rate-limited by the Newtonian viscosities of the dispersed and continuous phases. For analysis, a least squares technique is developed which, compared to the more common boundary integral methods, is simpler to implement and especially suited for systems where one liquid is much more viscous than the other (i.e., when the viscosity ratio lambda, defined as the ratio of the dispersed to continuous phase viscosities, approaches either zero or infinity). To demonstrate the validity of the proposed least squares technique, its results are shown to agree well with boundary integral calculations for moderate values of lambda, and with experimental data when lambda is much larger than unity (approximately 10(6)). Predictions at infinite viscosity ratio--the regime in which the least squares technique is most useful--are then used to evaluate interfacial tensions associated with a system of practical importance, namely, the dispersion of heavy crude oil in an aqueous environment. This amounts to a novel and accurate technique for determining interfacial tensions--especially those of low values (1 mN/m or less)--between density-matched liquids where at least one of the phases is highly viscous. The experimental part of this study involves the use of suction pipettes to manipulate the shapes of individual micrometer-sized droplets, thus avoiding the need for complex flow-generating devices to create drop deformations.     

Band texture formation of sheared polymeric liquid crystalline state

The phenomenon of band texture formation of sheared main chain liquid crystalline polymers is reviewed. The bands seen in a polarizing microscope are optical effects. The macromolecular chains are aggregated into zig-zag bent fibrils perpendicular to the bands. The band texture is formed during shear relaxation. The induction period depends on the shear rate applied, the shearing time, solution concentration (lyotropic), solution layer thickness, temperature and the nature of the polymer. There exists a critical shear deformation to bring a multi-domain nematic or cholesteric phase into a monodomain continuous phase, from which the band texture is formed. These two phases show quite different rheological behavior. In certain cases randomly oriented regions of bands can also be formed during quenching of a thermotropic nematic polymer melt or during standing of a lyotropic nematic polymer solution, where the nematic domains in the melt or in the solution have grown to a sufficient size.     

Flow of liquid around the encapsulated drop of another liquid

The problem of the infinite uniform flow of liquid around the spherical drop coated with the porous layer is solved. External liquid permeates into the porous layer but is not mixed with the liquid located in the internal cavity of a capsule. The flow inside the porous layer is described by the Brinkman equation; moreover, the viscosity of the Brinkman medium is assumed to be different than the viscosity of pure liquid. The boundary condition of the jump of tangential stresses at the liquid-porous medium interface is used. Velocity and pressure distributions are found and the hydrodynamic force acting on the capsule is calculated. Different limiting cases are considered.     

First observation of electromechanical effects in a chiral ferroelectric columnar liquid crystal

Electromechanical (converse piezoelectric) responses of an electrically switchable chiral ferroelectric columnar liquid crystal 1,2,5,6,8,9,12,13-octakis\[(S)-2-heptyloxy] dibenzo \[ e , l] pyrene, were studied under a.c. electric fields. The liquid crystal phase has a C2 rotational symmetry, the same as that of SmC* liquid crystals or of Rochelle Salt, but the responses are orders of magnitude weaker. The possible physical reasons for the observed weak mechanical responses and, in view of the results, the switching mechanism are discussed.     

Electrophoretic mobility of a liquid drop in a salt-free medium

A theory is proposed for the electrophoretic mobility mu of dilute spherical liquid drops of radius a in salt-free media containing only counterions (e.g., nonaqueous media). As in the case of the electrophoretic mobility of rigid particle in salt-free media, there is a certain critical value of the drop surface charge separating two cases, that is, the low-surface-charge case and the high-surface-charge case. For the low-surface-charge case, mu coincides with that of a drop in an electrolyte solution in the limit of very low electrolyte concentrations kappaa-->0 (Hückel's limit), where kappa is the Debye-Hückel parameter. For the high-surface-charge case, however, mu becomes constant independent of the drop surface charge, since the counterion condensation takes place near the drop surface.     

Experimental evidence of the motion of a single out-of-equilibrium drop

We show experimentally that when a single, neutrally buoyant drop is injected into a binary mixture either it remains quiescent or it moves, depending on whether the composition of the drop and that of the surrounding phase coincide with the equilibrium concentrations. In general, the movement of out-of-equilibrium drops, which is called diffusiophoresis, is induced by the Korteweg body force. This force is proportional to the chemical potential gradient and is therefore nonzero only when the system is in chemical nonequilibrium. In this letter, we show experimentally that this movement occurs for a single drop as well, even when the initial condition is (almost) isotropic. This instability, although it does not have a complete analytical explanation, has been predicted in the numerical simulations by Vladimirova et al. (Vladimirova, N.; Malagoli, A.; Mauri, R. Phys. Rev. E 1999, 60, 2037).     

Experimental investigation of the concept of molecular migration within sheared polystyrene

Several important aspects of the flow in polymer melts through capillaries remain unexplored. This paper examines experimentally one such effect associated with the radial shear-stress gradient in capillaries. During capillary melt flow of a polymer with a wide molecular weight distribution, migration of the large molecules away from the region of highest shear stress, i.e., at the capillary wall, has been predicted but only modestly investigated. This effect has the potential to produce a molecular weight spectrum over the cross section of extruded polymer. Studies of distribution in shear were conducted on a well-characterized wide-distribution polystyrene (M?_w = 234,000). An Instron Rheometer equipped with a long capillary (length/diameter ratio of 66.7) was used to perform the extrusion at temperatures of 160–250°C. A solvent coring procedure was used to dissolve away concentric layers of polymer from the extrudate for molecular weight analyses. The method has been shown to cut clean sections without selective extraction. Values of M?_w, M?_n and M?_w/M?_n were calculated from complete molecular weight distribution data obtained by calibrated gel permeation chromatography. For a wide range of shear rates and temperatures, no evidence for molecular fractionation was observed. Shear degradation of this polymer was found to be small. However, at high shear rates at 250°C, evidence indicating extensive shear-induced thermal degradation was found. No evidence for oxidative degradation at the extrudate surface was found at either low or high shear rates at this temperature.     

Experimental evidence of the influence of the surface chemistry of the packing material on the column pressure drop in reverse-phase liquid chromatography

The permeabilities of six columns packed with different packing materials (neat silica, C(1) endcapped silica at 3.92 micro mol/m(2), C(18) bonded and endcapped silica with 0.42, 1.01, 2.03, and 3.15 micro mol/m(2) of C(18) bonded chains) were measured. All these materials derive from the same batch of spherical particles, 5 micro m in diameter. The columns have the same tube inner diameter (phi=0.460+/-0.003 cm) and length (L=15.000+/-0.003 cm). The experimental conditions were the same, flow-rate (F(v)=1.000+/-0.003 mL/min) and temperature (295 K). Nevertheless, it was found that the column permeability decreases significantly, by about 25%, from the neat silica column to the one packed with the highest density of C(18)-bonded silica (3.15 micro mol/m(2)). The results measured on two duplicate columns were very reproducible. Accurate (+/-0.5 %) measurements of the hold-volumes with concentrated and dilute solutions of NO(3)(-) showed that the columns had all nearly the same external porosity. The result cannot be explained by the error made on the volume of the column tube either as it was measured accurately for all the columns. The residual explanation is that the interstitial velocity distribution between the packed particles depends on the chemical nature of the external surface of these particles.     

Wetting on the microscale: shape of a liquid drop on a microstructured surface at different length scales

Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nano- and micrometer-sized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1-2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars' height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air-water interfaces.     

Thermodynamic study of the liquid Fe-S system by use of a drop calorimeter

Heat contents of the Fe-S system were measured with a drop calorimeter in the sulfur composition range X_s = 0.380–0.500 (FeS) and in the temperature range 942–1506 K to construct an (H_T - H_298.15)-temperature-composition ternary diagram. By use of a thermodynamic analysis method, the mixing free energy, enthalpy and entropy of the liquid Fe-S mixtures were determined at 1473 and 1523 K, based on the measured heat contents. The partial molar free energies (activity of iron and partial pressure of diatomic sulfur) agreed well with the literature values, suggesting the applicability of the thermodynamic analysis method for liquid mixtures with semi-metals or chalcogen.     

Proton spin relaxation in a smectic liquid crystal

A proton spin relaxation study in the liquid crystal ethyl-[(methoxybenzylidene)-amino] cinnamate is presented. A “phase change” is observed at ≈ 103°C within the smectic A phase. Some liquid-like mobility exists below this temperature.     

A study of enthalpic relaxation of a liquid crystal

A liquid crystal, BL038, which was observed not to crystallize, has a glass transition at 215 K and a nematic to isotropic transition at 380 K. Samples aged below the glass transition at various temperatures T _a, exhibited an endotherm at the transition which developed with extent of ageing time, t _a. We attribute this endotherm to the relaxation of the glass towards the equilibrium liquid. The progress of the relaxation process was measured using differential scanning calorimetry. On subsequent reheating, the aged glass showed an apparent shift in the glass transition to higher temperatures. The endotherm was used to define the extent of enthalpic relaxation and the maximum value observed was found to increase initially then decrease, with the extent of undercooling from the glass transition temperature, Δ T, passing through a maximum for a Δ T = 15 K. From the temperature dependence of the relaxation times, an apparent activation enthalpy for the relaxation process of 85 ± 10 kJ mol^-1 was determined. The small value of the activation enthalpy compared with that found in the ageing of polymers reflects differences in the molecular species involved in relaxation processes.