Electrical conductance study of theta-liquid bridges

This work investigates the behavior of small liquid bridges that are formed between two horizontal supporting surfaces, aligned at the vertical direction. The contact lines of the liquid bridges are not edge-pinned but free to move across the supporting surfaces with the contact angle as a parameter (theta-bridges). An a.c. electrical conductance technique coupled with high resolution optical images is used to characterize the geometrical details of constant volume liquid bridges when their length is increased gradually until rupture. A mathematical framework is developed for the identification of the geometrical characteristics of theta-liquid bridges explicitly from conductance data. Theoretical predictions show good agreement with measurements for most of the bridge lengths (separation distance between supports) except close to the rupture point where the bridge is highly stretched. It is further shown that for short and moderate separation distances the present model can be used with confidence to determine the bridge volume and neck radius from the electrical signal.     

A conductance study of some cobalt(III) complexes in liquid ammonia

The electrical conductivity in liquid ammonia of Co(NH₃)₆(ClO₄)₃, Co(NH₃)₆(NO₃)₃, Co(NH₃)₅F(ClO₄)₂, Co(NH₃)₅F(NO₃)₂, and Co(NH₃)₅Cl(ClO₄)₂ has been measured between 2×10^?4 and 10^?2 M at temperatures varying from ?40 to ?75°C. In solutions more concentrated than 6×10^?4 M there is evidence that the only ionic species are univalent ions. The conductance data have been fitted to the Fuoss-Onsager-Skinner equation to give Λ₀ for the univalent complexes andK _A ¹ , the association constant of the uncharged complex ion pair. The quantities ΔH ^o and ΔS ^o were evaluated from the temperature dependence ofK _A ¹ . Walden products for the univalent complexes have been calculated and are discussed.     

A comparative study of void volume markers in immobilized-artificial-membrane and reversed-phase liquid chromatography

A comparative study of 10 void volume marker candidates on C(8), C(18), and IAM columns was described under several pH (3.0, 4.8, and 7.3) buffer conditions. Pycnometric analysis has been performed on the columns for comparison. In acidic conditions, small organic carboxylic acids suggested by the manufacturer did not necessarily give reliable and consistent estimates of the void volume for IAM column. However, changes in temperature and organic modifier composition led to small and predictable effects. The results presented here provide minimal but practical guidelines for the use of void volume markers in IAM chromatography.     

Enhancement of capillary forces by multiple liquid bridges

Capillary forces can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. In this paper, we study numerically the effect of meniscus size on the force between two homogeneous flat plates for different contact angles. The force distance curves show excellent quantitative agreement with previous investigations. The results for n menisci of equal total liquid volume reveal interesting scaling properties and an unexpected maximum force for moderately hydrophilic surfaces (i.e., contact angles around 70 degrees ). Further, we calculate the minimum solid-liquid area for multiple bridges, the cohesive stress (i.e., force per area) between the plates, and the work required to separate them. The results are presented in two-dimensional maps, which may be useful in the understanding of biological attachment structures and in the design of artificial contact systems.     

Probing non-polarizable liquid/liquid interfaces using scanning ion conductance microscopy

The study of microscopic structure of a liquid/liquid interface is of fundamental importance due to its close relation to the thermodynamics and kinetics of interfacial charge transfer reactions.In this article,the microscopic structure of a non-polarizable water/nitrobenzene(W/NB) interface was evaluated by scanning ion conductance microscope(SICM).Using SICM with a nanometer-sized quartz pipette filled with an electrolyte solution as the probe,the thickness of this type of W/NB interface could be measured at sub-nanometer scale,based on the continuous change of ionic current from one phase to another one.The effects for thicknesses of the non-polarizable W/NB interfaces with different electrolyte concentrations,the Galvani potentials at the interface and the applied potentials on the probe were measured and systematically analyzed.Both experimental setups,that is an organic phase up and an aqueous down,and a reverse version,were employed to acquire the approach curves.These data were compared with those of an ideal polarizable interface under the similar experimental conditions,and several characteristics of non-polarizable interfaces were found.The thickness of a non-polarizable interface increases with the decrease of electrolyte concentration and the increase of applied potential,which is similar to the situation of a polarizable liquid/liquid interface.We also find that the Galvani potential across a non-polarizable interface can also influence the interfacial thickness,this phenomenon is difficult to observe when using polarizable interface.Most importantly,by the comparison of two kinds of liquid/liquid interfaces,we experimentally proved that much more excess ions are gathered in the space charge layer of non-polarizable interfaces than in that of polarizable interfaces.These results are consistent with the predictions of molecular dynamic simulations and X-ray reflectivity measurements.     

Nanoscopic liquid bridges between chemically patterned atomistic walls

A binary liquid mixture, containing the Lennard-Jones molecules A and B, in equilibrium with a bulk liquid reservoir near the point of phase separation, confined between atomistic chemically patterned walls, is studied by grand canonical Monte Carlo simulations. In the bulk, the B-rich phase is stable and the A-rich phase is metastable. The walls bear patches attractive to A; when the walls are close, A-rich liquid bridges condense between the patches. The normal and lateral forces on the walls are measured as a function of the wall separation and of the lateral displacement between the patches on opposite walls. When there are one or two molecular layers in the bridge and the wall lattice constant is close to that of crystalline A, the normal and lateral forces depend strongly on the registry of the wall lattices, varying in an oscillatory manner.     

The strength of liquid bridges in random granular materials

The strength of capillary bridges in randomly packed granular media was analyzed by means of computer simulations. A novel simulation method, based on the tracking of moving interfaces, has been implemented and used for determining the equilibrium shape of capillary bridges in a granular medium under a range of liquid saturations and solid-phase geometry. The net force acting on each grain due to the capillary bridges was evaluated, as well as the aggregate force acting between two wet granular media during their separation in the normal directions. The simulation results are consistent with previous experimental observations and reveal interesting phenomena such as the existence of a maximum in the tensile strength of a wet granular medium as function of liquid saturation.     

Equilibrium shapes of liquid bridges under gravity: Symmetry breaking and imperfect bifurcations of two-dimensional bridges

The equilibrium shapes of a two-dimensional liquid bridge are constructed via a shooting and continuation numerical solution of the Laplace—Young equation which focuses on the bifurcation points of the solution branches. For the neutrally buoyant case, two new asymmetric shapes bifurcate from the point of maximum excess pressure where circular profiles with reflective symmetry have been shown to be unstable with respect to constant pressure disturbances. This occurs when the profile is exactly at right angles to the support. With the introduction of gravitational effects, this codimension 5 singularity is retained although the critical contact angle decreases slightly below 90° as one increases the difference between the interior and the exterior densities. However, when a slight tilt angle is introduced to the bridge, the maximum pressure singularity degenerates into two turning points (folds) and the solution branches are isolated from each other. This indicates that hysteretic jumps in the surface curvature and excess pressure exist with respect to changes in diameter/length ratio or liquid volume. Our numerical solution also reveals the existence of a maximum bridge volume that can be sustained by a nonneutrally buoyant bridge. No equilibrium shapes, symmetric or asymmetric, exist for bridge volumes beyond this critical value.     

Rupture kinetics of liquid bridges during a pulling process: a kinetic density functional theory study

Capillary bridge is a common phenomenon in nature and can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. Especially, it plays a crucial role in the operation of atomic force microscopy (AFM) and influences in the measured force. In the present work, we study the rupture kinetics and transition pathways of liquid bridges connecting an AFM tip and a flat substrate during a process of pulling the tip off. Depending on thermodynamic conditions and the tip velocity, two regimes corresponding to different transition pathways are identified. In the single-bridge regime, the initial equilibrium bridge persists as a single one during the pulling process until the liquid bridge breaks. While, in the multibridge regime the stretched liquid bridge transforms into an intermediate state with a collection of slender liquid bridges, which then break gradually during the pulling process. Moreover, the critical rupture distance at which the bridges break changes with the tip velocity and thermodynamic conditions, and its maximum value occurs near the boundary between the single-bridge regime and the multibridge regime, where the longest range capillary force is produced. In this work, the effects of tip velocity, tip size, tip-fluid interaction, and humidity on rupture kinetics and transition pathways are also systematically studied.     

The electrical conductance of hexaamminemetal nitrates in liquid ammonia

The electrical conductance behavior of hexaamminemetal nitrates of Cr(III), Co(II–III), Ni(II), and Cu(II) in liquid ammonia has been measured at temperatures varying from –40 to –75°C. Both univalent and divalent dissociations have been considered. The two outer-sphere association constantsK ₁ andK ₂, which control the properties in the concentration range employed, have been determined together with ₀ in a way described earlier by Fuoss. The thermodynamic quantities H^o and S^o of the univalent dissociation, evaluated from the temperature dependence of the association constants, are remarkably independent of the nature and charge of the metal ion. Walden products of the cations are presented and discussed.     

Stretching liquid bridges with bubbles: the effect of air bubbles on liquid transfer

Liquid bridges containing bubbles are relevant to industrial printing and are also a topic of fundamental scientific interest. We use flow visualization to study the stretching of liquid bridges, both with and without bubbles, at low capillary numbers. We find that whereas the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force air bubbles selectively toward the least hydrophilic plate confining the liquid bridge.     

Controlling the formation of capillary bridges in binary liquid mixtures

We study the formation of capillary bridges between micrometer-sized glass spheres immersed in a binary liquid mixture using bright field and confocal microscopy. The bridges form upon heating due to the preferential wetting of the hydrophilic glass surface by the water-rich phase. If the system is cooled below the demixing temperature, the bridges disappear within a few seconds by intermolecular diffusion. Thus, this system offers the opportunity to switch the bridges on and off and to tune precisely the bridge volume by altering the temperature in a convenient range. We measure the bridge geometry as a function of the temperature from bright field images and calculate the cohesive force. We discuss the influence of the solvent composition on the bridge formation temperature, the strength of the capillary force, and the bridge volume growth rate. Furthermore, we find that the onset of bridge formation coincides with the water-lutidine bulk coexistence curve.     

Experimental studies of the liquid junction potential between electrolyte solutions in different solvents part IV. A study using mixed-solvent salt bridges

The problem of the liquid junction potential (ljp) between different solvents was studied by measuring the emf's of a cell with a mixed-solvent salt bridge: Ag65 mM AgClO₄, 5 mM Et₄NClO₄ (S₁);1 mM Et₄ NClO₄ (S₁); 0.1 M MX (S₃ + S′₃); 1 mM Et₄NClO₄ (S₂);5 mM Et₄NClO₄, 5 mM AgClO₄ (S₂)6Ag. For 130 different combinations of solvents S₁, S₂, and (S₃+S′₃), the emf's were measured by changing the composition of (S₃ + S′₃) from pure S₃ to S′₃. Though there were exceptions, the emf's in many cases changed linearly or nearly linearly with the volume fraction of the mixed solvents. It was considered that the results were obtained because the component of the Ijp which was due to the solvent-solvent interactions at the mixed solvent/pure solvent junctions often changed in linear or near-linear relations with the volume fraction of the mixed solvent. A preliminary experiment showed some possibility of similar relations also to hold for mixed solvent/mixed solvent junctions.     

A Monte Carlo study of liquid benzene

A Monte Carlo study for liquid benzene, where the pair potential is described as a sum of Lennard-Jones (12, 6) interactions acting between six equivalent centres of each molecule, has been carried out. The potential parameters have been chosen in such a way that the internal energy and virial pressure were in agreement with the experimental values. The liquid structure has been examined in detail by computing several distribution functions: the radial, the angular correlations and the radial-angular distribution between molecular planes.     

Effects of soluble surfactants on the deformation and breakup of stretching liquid bridges

Surfactants are routinely used to control the breakup of drops and jets in many applications such as inkjet printing, crop spraying, and DNA or protein microarraying. The breakup of surfactant-free drops and jets has been extensively studied. By contrast, little is known about the closely related problem of interface rupture when surfactants are present. Solutions of a nonionic surfactant, pentaethylene glycol monododecyl ether, or C12E5, in water and in 90 wt % glycerol/water are used to show the effects of surfactant and viscosity on the deformation and breakup dynamics of stretching liquid bridges. Equilibrium surface tensions for both solutions can be fitted with the Langmuir-Szyskowski equation. All experiments have been done at 24 degrees C. The critical micelle concentrations for C12E5 are 0.04 and 0.4 mM in water and the glycerol/water solution, respectively. With high-speed imaging, the dynamic shapes of bridges held captive between two rods of 3.15 mm diameter are captured and analyzed with a time resolution of 0.1-1 ms. The bridge lengths are 3.15 mm initially and about 5-7 mm at pinch-off. Breakup occurs after stretching for about 0.2-0.3 s, depending on the solution viscosity and the surfactant concentration. When the liquid bridges break up, the volume of the sessile drop left on the bottom rod is about 3 times larger than that of the pendant drop left on the top rod. This asymmetry is due to gravity and is influenced by the equilibrium surface tensions. Surfactant-containing low-viscosity water bridges are shown to break up faster than surfactant-free ones because of the effect of gravity. With or without surfactant, water bridges form satellite drops. Surfactant-containing high-viscosity glycerol/water bridges break up more slowly than surfactant-free ones because of strong viscous effects. Moreover, the shapes of the sessile drops close to breakup exhibit a "pear-like" tip; whether a satellite forms depends on the surface age of the bridge before stretching commences. These unexpected effects arising from the addition of surfactants are due to the capillary pressure reduction and Marangoni flows linked to dynamic surface tension.     

Shearing of nanoscopic bridges in two-component thin liquid layers between chemically patterned walls

Bridge phases associated with a phase transition between two liquid phases occur when a two-component liquid mixture is confined between chemically patterned walls. In the bulk the liquid mixture with components A, B undergoes phase separation into an A-rich phase and a B-rich phase. The walls bear stripes attractive to A. In the bridge phase A-rich and B-rich regions alternate. Grand canonical Monte Carlo studies are performed with the alignment between stripes on opposite walls varied. Misalignment of the stripes places the nanoscopic liquid bridges under shear strain. The bridges exert a Hookean restoring force on the walls for small displacements from equilibrium. As the strain increases there are deviations from Hooke's law. Eventually there is an abrupt yielding of the bridges. Molecular dynamics simulations show the bridges form or disintegrate on time scales which are fast compared to wall motion and transport of molecules into or from the confined space. Some interesting possible applications of the phenomena are discussed.     

A New optical liquid membrane study technique : II. A Holographic interferometric study of transport through a liquid membrane

A liquid membrane consisting of a crown ether carboxylic acid dissolved in chloroform placed between a basic source aqueous solution and an acidic receiving aqueous solution was observed in a holographic interferometer to transport potassium ion by coupled counter-ion transport. Steady-state transport was achieved and quite precise measurements of the rate of transport made. The evolution of concentration gradients as a function of time was directly observed.     

First-principles study of length dependence of conductance in alkanedithiols

Electronic transport properties of alkanedithiols are calculated by a first-principles method based on density functional theory and nonequilibrium Green's function formalism. At small bias, the I-V characteristics are linear and the resistances conform to the Magoga's exponential law. The calculated length-dependent decay constant gamma which reflects the effect of internal molecular structure is in accordance with most experiments quantitatively. Also, the calculated effective contact resistance R(0) is in good agreement with the results of repeatedly measuring molecule-electrode junctions [B. Xu and N. Tao, Science 301, 1221 (2003)].     

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.