Model for drop coalescence in a locally isotropic turbulent flow field

The proposed model views drop coalescence in a turbulent flow field as a two-step process consisting of formation of a doublet due to drop collisions followed by coalescence of the individual droplets in a doublet due to the drainage of the intervening film of continuous phase under the action of colloidal (van der Waals and electrostatic) and random turbulent forces. The turbulent flow field was assumed to be locally isotropic. A first-passage-time analysis was employed for the random process of intervening continuous-phase film thickness between the two drops of a doublet in order to evaluate the first two moments of coalescence-time distribution of the doublet. The average drop coalescence time of the doublet was dependent on the barrier for coalescence due to the net repulsive force (net effect of colloidal repulsive and turbulent attractive forces). The predicted average drop coalescence time was found to be smaller for larger turbulent energy dissipation rates, smaller surface potentials, larger drop sizes, larger ionic strengths, and larger drop size ratios of unequal-sized drop pairs. The predicted average drop coalescence time was found to decrease whenever the ratio of average turbulent force to repulsive force barrier became larger. The calculated coalescence-time distribution was broader, with a higher standard deviation, at lower energy dissipation rates, higher surface potentials, smaller drop sizes, and smaller size ratios of unequal drop pairs. The model predictions of average coalescence-rate constants for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) in a high-pressure homogenizer agreed fairly well with the inferred experimental values as reported by Narsimhan and Goel (J. Colloid Interface Sci. 238 (2001) 420-432) at different homogenizer pressures and SDS concentrations.     

Interfacial deformation between an impacting water drop and a silicone-oil surface

Detailed observations have been conducted on the interfacial deformation of a silicone oil surface and a water drop falling on it. Eleven kinds of silicone oils with wide variations of kinematic viscosity, nu(T)=1-10(5) mm(2)/s, have been tested. The oil surface is disturbed by a water drop with a diameter d(L)=3.1 mm, which falls freely on it from a height of 100-1000 mm. Special attention has been directed to the maximum depth of the cavity formed on the oil surface D(M) and to the maximum diameter of the water drop spreading on the oil surface d(M). We have categorized the configurations of the oil cavity into seven patterns, and those of the water drop at the oil-water interface into five patterns. The maximum cavity depth D(M)/d(L) can be well correlated by a dimensionless group Re(TL)We(TL), where Re(TL) is Reynolds number based on d(L) and nu(T) and We(TL) is Weber number with the water density and surface tension of oil. The maximum diameter of the impacting water drop d(M)/d(L) can be correlated by the Reynolds number with a viscosity of water (Re(L)) and the Ohnesorge number (Oh). Moreover, the condition under which the impacting water drop is smashed into pieces has been also examined based on Re(L) and Oh.     

Rheology of a thermotropic polyester in the nematic and isotropic states

The rheological properties of a thermotropic polyester were determined in the nematic and isotropic states. In the isotropic state, the viscosity is almost constant and the polymer is only slightly elastic. The nematic phase has a lower viscosity than the isotropic, except at low frequencies or shear rates, where the viscosity increases as though the polymer had a yield stress. There is a marked dependence of the rheology on shear history. The effects of shearing can be erased by returning the material first to the isotropic state and then back to the nematic state. The results are discussed with reference to analogous observations in small-molecule liquid crystals and in thermotropic aromatic co-polyesters.     

Influence of non-mesogenic impurities on a nematic to isotropic phase transition

Non-mesomorphic solutes depress the normal nematic-isotropic transition temperature in liquid crystals. When non-mesomorphic solutes are added to a nematic liquid crystal, the nematic-isotropic transition temperature is depressed and a two phase region is formed due to the presence of impurities of the solutes. The present paper explains the formation of this two phase region by the Landau-de Gennes phenomenological theory, which agrees fairly well with the experimental observations. We also note that this two phase region indicates the tricritical behaviour of the nematic-isotropic phase transition and the phase diagram near the tricritical point is also obtained.     

Suspended,one-side anchored,or double-side anchored nematic droplets in an isotropic medium

Nematic droplets in an isotropic liquid crystal provide an interesting geometrical environment for director alignment, depending on the type of surface used. We prepared three types of substrates with nematic-philic, nematic-phobic, and neutral surfaces. By choosing the proper surface type, we were able to selectively produce three types of droplets (suspended, one-side anchored, and double-side anchored) near the phase transition temperature. These droplets exhibited single-polar or bipolar defects depending on the anchoring status. The one-side anchored and the suspended droplets underwent anchoring transitions in one-step and in two-steps processes on cooling, respectively. A hybrid cell with two types of substrates exhibited a truncated cone-shaped droplet, and two nearby cone-shaped droplets produced a stable doule-curved surface geometry with a saddle point of – 1 defect between isotropic-nematic boundaries. The mergence of these droplets was also investigated.     

Diffusion of spheres in isotropic and nematic suspensions of rods

Diffusion of a small tracer sphere (apoferritin) in isotropic and nematic networks [of fd virus] is discussed. For a tracer sphere that is smaller than the mesh size of the network, screened hydrodynamic interactions between the sphere and the network determine its diffusion coefficient. A theory is developed for such interactions as well as their relation to the long-time self-diffusion coefficient. Fluorescence correlation spectroscopy measurements on mixtures of apoferritin and fd virus are presented. The long-time self-diffusion coefficient of apoferritin is measured as a function of the fd-virus concentration, both in the isotropic and nematic state, in directions parallel and perpendicular to the nematic director. The hydrodynamic screening length of the fd-virus network as a function of fd concentration is obtained by combining these experimental data with the theory. Surprisingly, the screening length increases with increasing concentration in nematic networks. This is due to the increase in the degree of alignment, which apparently leads to a strong increase of the screening length. Hydrodynamic screening is thus strongly diminished by alignment. A self-consistent calculation of the screening length does not work at higher concentrations, probably due to the strong variation of the typical incident flow fields over the contour of a rod.     

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane-azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane-azomethine polyether at cooling rates of 10 and 20°C min^-1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time-dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.     

Experimental investigation on thermocapillary drop migration at large Marangoni number in reduced gravity

Results from a space experiment on thermocapillary drop migration conducted on board the Chinese spacecraft ShenZhou-4 are presented in this paper. In the experiment, isolated drops of Fluorinert liquid moved in a matrix liquid of 5cst silicone oil at values of the Marangoni numbers (Ma) ranging up to 5500 and the interferometry images showed the temperature distribution inside the test cell. The drop migration velocity was measured. The experimental results show that the scaled drop migration velocity V/V(YGB) obviously decreases with Ma increasing the values up to 5500. The space experimental results are also compared with those from our early experiments, other space experiments, and some theoretical predictions.     

Effect of persistent flexibility on the isotropic, nematic and columnar ordering in a self-assembling system

Concentrated solutions of reversibly assembling amphiphilic molecules often exhibit a variety of liquid-crystalline mesophases due to the asymmetry of their aggregates. For strongly elongated rod-like micelles flexibility may significantly influence the relative stability of these phases. This question is addressed for a simple model for a self-assembling system of monodisperse linear rod-like aggregates which are considered to be persistent flexible. Analogously to a recent calculation for a self-assembling system in which the rod-like aggregates are assumed to be perfectly rigid, persistent flexible aggregates may exhibit the same isotropic-nematic-columnar progression of phases. However, increasing the flexibility reduces the concentration range over which the nematic phase is stable. For sufficiently flexible aggregates the nematic phase is abandoned altogether and a direct isotropic-columnar transition occurs.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

Abstract

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

On the observation of flow-induced order in the isotropic phase of a nematic discogen

The phenomenon of flow-induced order observed in the isotropic phase of nematic discotic materials is studied and the orientation of the induced director explained by the existence of the molecules in short columns. The occurrence of this order in both the isotropic and nematic phases explains the low transition enthalpy, agreeing with the inferences made in the literature.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Instabilities across the isotropic conductivity point in a nematic phenyl benzoate under AC driving

We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy epsilona and an inversion temperature for electrical conductivity anisotropy sigmaa, exhibits a monostable tilted alignment near TIN, the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative sigmaa and homeotropic alignment, and (ii) weakly positive sigmaa and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Carr-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.     

Effect of non-homogeneous surface viscosity on the Marangoni migration of a droplet in viscous fluid

Marangoni migration of a single droplet in an unbounded viscous fluid under the additional effect of variable surface viscosity is studied. The surface tension and the surface viscosity depend on concentration of dissolved species. Cases of the motion induced by the presence of a point source and by a given constant concentration gradient are considered. The dependence of the migration velocity on the governing parameters is computed under quasi-stationary approximation. The effect of weak advective transport is studied making use of singular perturbations in the Peclet number, Pe. It is shown that, when the source is time dependent a Basset-type history term appears in the expansion of the concentration and, as a result, the leading order correction to the flow and to the migration velocity is of O(Pe(1/2)). If the source of active substance driving the flow is steady, the effect of convective transport on the migration is weaker.     

The "Wimple": rippled deformation of a fluid drop caused by hydrodynamic and surface forces during thin film drainage

It is well-known that hydrodynamic pressures in a thin draining liquid film can cause inversion of the curvature of a drop or bubble surface as it approaches another surface, creating a so-called "dimple". Here it is shown that a more complicated rippled shape, dubbed a "wimple", can be formed if a fluid drop that is already close to a solid wall is abruptly pushed further toward it. The wimple includes a central region in which the film remains thin, surrounded by a ring of greater film thickness that is bounded at the outer edge by a barrier rim where the film is thin. This shape later evolves into a conventional dimple bounded by the barrier rim, which then drains in the normal way. During the evolution from wimple to dimple, some of the fluid in the thicker part of the film ring flows toward the central region before eventually draining in the opposite direction. Although the drop is pressed toward the wall, the central part of the drop moves away from the wall before approaching it again. This is observed even when the inward push is too small to create a wimple.     

Dielectric and viscous properties of 6CHBT in the isotropic and nematic phases

This paper presents the results of measurements of the principal electric permittivities ε ∥_*(T, ω and ε⊥*(T, ω) and the viscosity for 4-(trans-4-n-hexylcyclohexyl)isothiocyanatobenzene (C₆H₁₃-CyHx-Bz-N=C=S, 6CHBT). In the nematic phase, the Miesowicz η₂ viscosity coefficient was measured in a sample oriented due to the flow. On the basis of the temperature dependence of the static permittivities, using the Maier-Meier equations, the angle β between the dipole moment vector and the long axis of the 6CHBT molecule, the square of the molecular apparent dipole moment μ² _app ² and the nematic order parameter S(T), were determined. From the temperature dependence of the viscosity and the relaxation time corresponding to the molecular rotation around the short axis, the strength of the nematic potential and the effective length of the 6CHBT molecule (in the isotropic phase) were estimated.     

The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

Abstract

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T _IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.     

The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T_IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.