Growth dynamics of a liquid crystal at the three- to two-dimensional crossover in a hele-shaw cell

The radial growth of a cholesteric liquid crystal phase was studied experimentally in a thick Hele-Shaw cell. In the experiments, nucleation and growth of the cholesteric phase occurs after a temperature quench from the isotropic liquid. The growth process changes from three- to two-dimensional when the growing phase touches the substrates of the Hele-Shaw cell. It was found that the growth rate generally increases at the dimensional crossover. Also, this increase in growth rate is approximately constant, independent of the depth of the temperature quench. Possible causes for the observed experimental behavior are discussed.     

Diffusion driven instability to a drift driven one: Turing patterns in the presence of an electric field

We report a general formula for the critical electric field required to trigger a pattern formation in a Turing system in the presence of an electric field (drift term). Our result encompasses all situations from pure diffusion to pure drift.     

Response mechanism of a vertical alignment mode driven by fringe-field switching

The response mechanism of a vertical alignment mode, driven by a fringe field, is investigated in detail using small-angle approximation. The flow effects can be ignored when using theoretical analysis. The period of the liquid crystal (LC) deformation in the transversal direction, instead of the lognitudinal direction, shows the cell gap effect on the response time in the LC layer's thickness. The authors' analytical results indicate that a liquid crystal display (LCD) mode with a small transversal period could provide a new method that gives a fast response.     

Cross-streamline migration of a semiflexible polymer in a pressure driven flow

Experiments and simulations on single α-actin filaments in the Poiseuille flow through a microchannel show that the center-of-mass probability density across the channel assumes a bimodal shape as a result of pronounced cross-streamline migration. We reexamine the problem and perform Brownian dynamics simulations for a bead-spring chain with bending elasticity. Hydrodynamic interactions between the pointlike beads are taken into account by the two-wall Green tensor of the Stokes equations. Our simulations reproduce the bimodal distribution only when hydrodynamic interactions are taken into account. Numerical results on the orientational order of the end-to-end vector of the model polymer are also presented together with analytical hard-needle expressions at zero flow velocity. We derive a Smoluchowski equation for the center-of-mass distribution and carefully analyze the different contributions to the probability current that causes the bimodal distribution. As for flexible polymers, hydrodynamic repulsion explains the depletion at the wall. However, in contrast to flexible polymers, the deterministic drift current mainly determines migration away from the centerline and thereby depletion at the center. Diffusional currents due to a position-dependent diffusivity become less important with increasing polymer stiffness.     

DC-biased AC-electrokinetics: a conductivity gradient driven fluid flow

This paper studies the principles of fluid flow manipulation based on DC-biased AC-electrokinetics. This method makes use of planar parallel electrodes in a microfluidic channel in contact with an electrolyte solution, with a DC biased AC electrical signal applied to the electrode pair. Due to the application of DC bias, incipient Faradaic electrolytic reactions take place resulting in an increase of the ionic content of the bulk solution. The ionic content was found to be dissimilar at the cathodic and anodic sides of the channel and a conductivity difference of approximately 10% was measured for 2 V(DC). Fluid flow is generated by the action of the DC biased AC electric signal acting on the transverse conductivity gradient generated across the microchannel. The induced flow in the form of vortex was characterized experimentally and the results substantiated theoretically. The velocity of the induced flow vortex under the employed experimental conditions was ~600 to 700 μm s(-1) which is faster than those obtained in conventional AC-electroosmosis and AC-electrothermal types of flows.     

Superspreading driven by Marangoni flow

The spontaneous spreading (called superspreading) of aqueous trisiloxane ethoxylate surfactant solutions on hydrophobic solid surfaces is a fascinating phenomenon with several practical applications. For example, the ability of trisiloxane ethoxylate surfactants to enhance the spreading of spray solutions on waxy weed leaf surfaces, such as velvetleaf (Abutilion theophrasti), makes them excellent wetting agents for herbicide applications. The superspreading ability of silicone surfactants has been known for decades, but its mechanism is still not well understood. In this paper, we suggest that the spreading of trisiloxane ethoxylates is controlled by a surface tension gradient, which forms when a drop of surfactant solution is placed on a solid surface. The proposed model suggests that, as the spreading front stretches, the surface tension increases (the surfactant concentration becomes lower) at the front relative to the top of the droplet, thereby establishing a dynamic surface tension gradient. The driving force for spreading is due to the Marangoni effect, and our experiments showed that the higher the gradient, the faster the spreading. A simple model describing the phenomenon of superspreading is presented. We also suggest that the superspreading behavior of trisiloxane ethoxylates is a consequence of the molecular configuration at the air/water surface (i.e. small and compact hydrophobic part), as shown by molecular dynamics modeling. We also found that the aggregates and vesicles formed in trisiloxane solutions do not initiate the spreading process and therefore these structures are not a requirement for the superspreading process.     

Spontaneous mode-selection in the self-propelled motion of a solid/liquid composite driven by interfacial instability

Spontaneous motion of a solid/liquid composite induced by a chemical Marangoni effect, where an oil droplet attached to a solid soap is placed on a water phase, was investigated. The composite exhibits various characteristic motions, such as revolution (orbital motion) and translational motion. The results showed that the mode of this spontaneous motion switches with a change in the size of the solid scrap. The essential features of this mode-switching were reproduced by ordinary differential equations by considering nonlinear friction with proper symmetry.     

Effect of vertical temperature variation on the oscillatory wetting instability in a fluid Ga-Pb alloy

Oscillatory wetting instabilities driven by capillary-gravitation forces have been explored very recently in the binary fluid Ga-Pb alloy [A. Turchanin, R. Tsekov and W. Freyland, J. Chem. Phys., 2004, 120, 11 171]. This system is characterized by a complete wetting transition at liquid-liquid coexistence. Due to its metallic nature the bulk and interfacial instabilities are strongly coupled via variation of the respective emissivities. In our previous work we have investigated these phenomena at different cooling cycles and at constant temperature inside the miscibility gap. In this study we present for the first time the observations of the oscillatory wetting instabilities also in heating cycles. The interfacial properties of a Ga0.95Pb0.05 alloy at conditions inside the miscibility gap have been investigated by following the second harmonic generation (SHG) intensity changes. Corresponding model calculations of the Pb-rich wetting film instabilities have been performed taking into account the effect of a temperature variation vertical to the bulk sample. The influence of this temperature variation on the occurrence of the oscillations is discussed.     

Transition from homogeneous Langmuir-Blodgett monolayers to striped bilayers driven by a wetting instability in octadecylsiloxane monolayers

We show that two dips of an oxidized silicon substrate through a prepolymerized n-octadecylsiloxane monolayer at an air-water interface in a rapid succession produces periodic, linear striped patterns in film morphology extending over macroscopic area of the substrate surface. Langmuir monolayers of n-octadecyltrimethoxysilane were prepared at the surface of an acidic subphase (pH 2) maintained at room temperature (22 +/- 2 degrees C) under relative humidities of 50-70%. The substrate was first withdrawn at a high dipping rate from the quiescent aqueous subphase (upstroke) maintained at several surface pressures corresponding to a condensed monolayer state and lowered soon after at the same rate into the monolayer covered subphase (downstroke). The film structure and morphology were characterized using a combination of optical microscopy, imaging ellipsometry, and Fourier transform infrared spectroscopy. An extended striped pattern, perpendicular to the pushing direction of the second stroke, resulted for all surface pressures when the dipping rate exceeded a threshold value of 40 mm min(-1). Below this threshold value, uniform deposition characterizing formation of a bimolecular film was obtained. Under conditions that favored striped deposition during the downstroke through the monolayer-covered interface, we observed a periodic auto-oscillatory behavior of the meniscus. The stripes appear to be formed by a highly correlated reorganization and/or exchange of the first monolayer, mediated by the Langmuir monolayer at the air-water interface. This mechanism appears distinctly different from nanometer scale stripes observed recently in single transfers of phospholipid monolayers maintained near a phase boundary. The stripes further exhibit wettability patterns useful for spatially selective functionalization, as demonstrated by directed adsorptions of an organic dye (fluorescein) and an oil (hexadecane).     

Synthesis of a three-member array of cycloadducts in a glass microchip under pressure driven flow

The synthesis of a series of cycloadduct products carried out in a glass microchip under pressure driven flow is presented. Initially, four different compounds were synthesized individually with conversions similar to those obtained in the corresponding batch macroscale reactions. Using identical experimental conditions for all four compounds, the results were highly predictable and reproducible, without the need for individual optimization. A multi-reaction experiment was then carried out in a single chip achieving the synthesis of a three-member array in a single run.     

Electrokinetically driven micro flow cytometers with integrated fiber optics for on-line cell/particle detection

This paper presents an innovative micro flow cytometer which is capable of counting and sorting cells or particles. This compact device employs electrokinetic forces rather than the more conventional hydrodynamic forces technique for flow focusing and sample switching, and incorporates buried optical fibers for the on-line detection of cells or particles. This design approach results in a compact microfluidic system and an easier integration process. The proposed cytometer integrates several critical modules, namely electrokinetic-focusing devices, built-in control electrodes, buried optical fibers for on-line detection, and electrokinetic flow switches for bio-particle collection. A linear relationship exists between the focused stream width (d) and the focusing ratio (F/φ), which is estimated to be D≈134.5−53.8F/φ. The relationship between the particle velocity (U) and the applied voltage (V) is also investigated. Numerical and experimental data confirm the effectiveness of the device when applied to the counting and sorting of 10 μm diameter particles and red blood cells.     

Steering population flow in coherently driven lossy quantum ladders

We present a detailed theory of a technique for the adiabatic control of the population flow through a preselected decaying excited level in a three-level ladder quantum system, as was experimentally demonstrated recently by Garcia-Fernandez et al. [Phys. Rev. Lett. 95, 043001 (2005)]. Specifically, we consider a three-state excitation chain of bound states, 1-2-3, of successively increasing excitation energy, in which probability loss via fluorescence occurs from states 2 and 3. We describe a laser excitation scheme that can, by adjustment of laser parameters, alter at will the relative fraction of population that, starting from state 1, is ultimately lost through states 2 and 3. We present analytical results for the conditions under which quasiadiabatic passage can take place.     

Formation of nanowire striations driven by Marangoni instability in spin-cast polymer thin films

Parallel striations made of silver nanowires were formed through the Marangoni instability induced during spin casting of poly(2-vinyl pyridine)/silver nanowire/chloroform solutions. The striation patterns of the silver nanowires resembled those obtained from spin casting of the corresponding neat polymer solutions, indicating essentially the same driving mechanism (i.e., the Marangoni instability). The silver nanowires were found to concentrate in the valleys of the striation pattern to balance the nonuniform surface tension distribution in the polymer thin film. The resulting nanowire striation patterns were found to depend on polymer concentration, rotational speed, and nanowire loading. Interestingly, this nanowire striation phenomenon was found to be independent of the substrate characteristics, hydrophobic or hydrophilic.     

Ferroelectricity and pressure-induced phenomena driven by neutral ionic valence instability of acid-base supramolecules

Supramolecular ferroelectric cocrystals of phenazine (Phz) with chloranilic acid (H(2)ca), bromanilic acid (H(2)ba), and fluoranilic acid (H(2)fa) have been characterized by the interplay between their structural transformations and solid-state acid-base (proton transfer) reactions. At ambient pressure, the Phz-H(2)ca, Phz-H(2)ba, and their deuterated crystals exhibit incomplete proton displacement, which transforms the neutral molecules into semi-ionic at low temperatures below the Curie point (T(c)(IC) < T < T(c)(I)). For the cocrystal of the less acidic H(2)fa, the ferroelectric phase is induced only by applying hydrostatic pressure above ~0.6 GPa. According to the combined studies of temperature-dependent dielectric permittivity and synchrotron X-ray diffraction, it was proved that the ferroelectric (FE-I) phase is always accompanied at lower temperatures by successive phase transitions to the lattice modulated phases with incommensurate periodicities (IC phase, T(c)(II) < T < T(c)(IC)) and with commensurate (2- or 3-fold) periodicities (FE-II or FE-III phase, T < T(c)(II)). Whereas the ground-state structures at ambient pressure are different from one another among the Phz-H(2)ca (FE-II form), Phz-H(2)ba (FE-III form), and Phz-H(2)fa (paraelectric form), their systematic changes under pressure depict a universal pressure-temperature phase diagram. The possible origins of structural changes are assigned to the valence instability and the frustrated Coulomb interactions that induce the charge disproportionation (coexisting neutral ionic) states with the staging spatial orders.     

Simultaneous patterning of nanoparticles and polymers using an evaporation driven flow in a vapor permeable template

Nanoparticles and polymers have great potential for lowering cost and increasing functionality of printed sensors and electronics. However, creation of practical devices requires that many of these materials be patterned on a single substrate, and many current patterning processes can only handle a single material at a time, necessitating alignment of serial processing steps. Higher throughput and lower cost can be achieved by patterning multiple materials simultaneously. To this end, the microfluidic molding process is adapted to pattern various nanoparticle and polymer inks simultaneously, in a completely additive manner, with three-dimensional control and high relative positional accuracy between the different materials. A differential template distortion observed in channels containing different inks is analyzed and found to result from pressure force in the template due to flow of a highly viscous and highly concentrated ink in small channels. The resulting optimization between patterning speed and dimensional fidelity is discussed.     

Bejan flow visualization of free convection in a Jeffrey fluid from a semi-infinite vertical cylinder

Journal of Thermal Analysis and Calorimetry - This article studies the pattern of heat lines in free convection non-Newtonian flow from a semi-infinite vertical cylinder via Bejan’s heat...     

Viscous fingering induced flow instability in multidimensional liquid chromatography

Viscous fingering is a flow instability phenomenon that results in the destabilisation of the interface between two fluids of differing viscosities. The destabilised interface results in a complex mixing of the two fluids in a pattern that resembles fingers. The conditions that enhance this type of flow instability can be found in coupled chromatographic separation systems, even when the solvents used in each of the separation stages have seemingly similar chemical and physical properties (other than viscosity). For example, the viscosities of acetonitrile and methanol are sufficiently different that instability at the interface between these two solvents can be established and viscous fingering results. In coupled chromatographic systems, the volume of solvent transported from one separation dimension to the second often exceeds the injection volume by two or more orders of magnitude. As a consequence, viscous fingering may occur, when otherwise following the injection of normal analytical size injection plugs viscous fingering would not occur. The findings in this study illustrate the onset of viscous fingering in emulated coupled chromatographic systems and show the importance of correct solvent selection for optimum separation performance.     

Pressure driven flow of polymer solutions in nanoscale slit pores

Polymer solutions subject to pressure driven flow and in nanoscale slit pores are systematically investigated using the dissipative particle dynamics approach. The authors investigated the effect of molecular weight, polymer concentration, and flow rate on the profiles across the channel of the fluid and polymer velocities, polymer density, and the three components of the polymers radius of gyration. They found that the mean streaming fluid velocity decreases as the polymer molecular weight and/or polymer concentration is increased, and that the deviation of the velocity profile from the parabolic profile is accentuated with increase in polymer molecular weight or concentration. They also found that the distribution of polymers conformation is highly anisotropic and nonuniform across the channel. The polymer density profile is also found to be nonuniform, exhibiting a local minimum in the center plane followed by two symmetric peaks. They found a migration of the polymer chains either from or toward the walls. For relatively long chains, as compared to the thickness of the slit, a migration toward the walls is observed. However, for relatively short chains, a migration away from the walls is observed.     

Heat transfer exaggeration and entropy analysis in magneto-hybrid nanofluid flow over a vertical cone: a numerical study

Journal of Thermal Analysis and Calorimetry - Entropy analysis is closely scrutinized for unsteady mixed convection in magneto-hybrid nanofluid (Cu–Fe $$_3$$ O $$_4$$ –water) flow over...