The effect of interface deformation due to gravity on line tension measurement by the capillary rise in a conical tube

Much research has been done on line tension measurement and interpretation, and attempts have also been made to measure line tension with simple, cheap and reliable methods that do not require excessive sophistication. Of particular interest is the method of determining line tensions of solid–liquid–vapor systems from the capillary rise in a conical tube. This simple and relatively inexpensive method gives line tension values comparable to those reported in the literature obtained via highly sophisticated instruments or techniques such as the well known axisymmetric drop shape analysis technique. The absolute value of line tension obtained using the conical tube method and assuming a spherical liquid–vapor interface is larger but of the same order of magnitude (1 μJ m⁻¹) as that reported in the literature. A theoretical analysis presented herein shows that by including the deformation of the liquid–vapor interface due to gravity in the conical capillary analysis, the line tension value inferred from the experimental data is reduced by approximately 50% and compares better with values in the literature obtained using other sophisticated methods. Thus a relatively simple, cheap, accurate and reliable method of line tension measurement has been advanced.     

The determination of interfacial tension by differential capillary rise

A method for directly determining interfacial tension without iteration from the difference in height between two capillaries is presented. An experimental technique is described for organic liquids both lighter and heavier than water, in which the menisci recede over surfaces wetted by the aqueous phase, thus providing the most favourable conditions for zero contact angle. The values determined experimentally using tables prepared from the known shapes of sessile interfaces agree well with accepted values of the interfacial tension.     

Dimensionless scaling methods for capillary rise

In this article the different dimensionless scaling methods for capillary rise of liquids in a tube or a porous medium are discussed. A systematic approach is taken, and the possible options are derived by means of the Buckingham π theorem. It is found that three forces (inertial, viscous and hydrostatic forces) can be used to obtain three different scaling sets, each consisting of two dimensionless variables and one dimensionless basic parameter. From a general point of view the three scaling options are all equivalent and valid for describing the problem of capillary rise. Contrary to this we find that for certain cases (depending on the time scale and the dominant forces) one of the options can be favorable. Individually the different scalings have been discussed and used in literature previously, however, we intend to discuss the three different sets systematically in a single paper and try to evaluate when which scaling is most useful. Furthermore we investigate previous analytic solutions and determine their ranges of applicability when compared to numerical solutions of the differential equation of motion (momentum balance).     

Wettability determined by capillary rise with pressure increase and hydrostatic effects

Capillary rise is the basis of some methods that are widely applied for the determination of contact angles as well as wettabilities of small particles. The equivalent hydraulic radius r(d) in the Classical Washburn equation is assumed to be particle-specific. But it seems that r(d) is not always constant when the type of liquids is different. The new equation with the pressure increment and the hydrostatic effects are theoretically derived based on the Washburn equation, so contact angles of small particles can be measured experimentally independently of r(d). The result shows the validity of the proposed method, and therefore, it becomes possible to accurately measure the wettability of small particles.     

On-line solution stability determination of pharmaceuticals by capillary electrophoresis

Summary Good agreement between the impurity levels in a batch of a related impurity of ranitidine were obtained by CE and HPLC. A solution of the impurity was positioned on the CE autosampler and analysed sequentially. The extent of degradation was monitored by loss of main peak and the formation of two principal degradation products. It was found that after 9.25 hours only 2% area/area of the original impurity remained. Buffering of the sample solution to pH 7 was shown to minimise this degradation.Unattended in-situ stability testing of an solution of the impurity in water was performed by CE.     

Effect of dynamic contact angle on capillary rise phenomena

Capillary rise experiments of different liquids in glass capillaries and in columns of packed powders were carried out. The analysis of this rise was performed according to the classical Washburn’s equation in which the calculation of a constant term is needed in order to be able to determine contact angle of the considered liquid on the capillary wall or powders. However, it was observed that this constant term apparently varies as a function of the liquid used, in contradiction with Washburn’s approach. A more fundamental study of alkane rise into glass capillaries was carried out showing that this apparent variation is due to the variation of contact angles, which can take large values (up to 60°) as a function of velocity of the liquid front, although their expected value is 0°. Therefore, in the case of powders, different approaches to determine the real constant term with respect to particle size are proposed. Consequently, the use of Washburn’s equation for the determination of contact angles of liquids on these powders is also discussed.     

Separation of inorganic anions in acidic solution by capillary electrophoresis

Inorganic anions are almost always determined by capillary electrophoresis (CE) at an alkaline pH, so the analytes will be fully ionized. However, a long-chain quaternary ammonium salt usually must be added as a flow modifier to the carrier electrolyte to reverse the direction of the electroosmotic flow. By working at a sufficiently acidic pH, the electroosmotic flow in fused-silica capillaries is virtually eliminated, and anions can be separated simply by differences in their electrophoretic mobilities. Excellent separations were obtained for AuCl₄⁻ and the chloro complexes of platinum group elements in HCl solution at pH 2.0 to 2.4. No additional buffer or flow modifier was needed. This CE technique is an excellent way to follow slow hydrolytic reactions in which one or more of the chloride ligands is replaced by water. Sharp peaks and good separations were also obtained for MnO₄⁻, VO₃⁻, chromate, molybdate, ferrocyanide, ferricyanide and stable complex ions such as chromium oxalate (CrO₃³⁻).     

Amplification of small analytes in polymer solution by capillary electrophoresis

We report concentration methods for the analysis of small solutes by capillary electrophoresis in conjunction with laser-induced native fluorescence using a Nd:YAG laser. After injecting samples, poly(ethylene oxide) (PEO) in the anode reservoir entered a capillary filled with Tris-borate buffer. When migrating in PEO solution, the analytes slowed down and stacked at the interface between the sample zone and PEO solution. As a result, the limits of detection (LODs) down to 8 pM for 2-naphthalenesulfonic acid and 70 pM for L-tryptophan have been achieved when injecting at 30 cm height for 120 s and 230 s, respectively. Such low LODs are partially due to the effects of NaCl in the samples and PEO on the fluorescence characteristics of the analytes. In addition, the concentrations of NaCl and PEO have great impacts on the migration of the analytes and electroosmotic flow, thereby affecting resolution and speed. Without pretreatment, the determinations of five important markers in urine samples and two acids in a cerebrospinal fluid sample have been performed separately, with the relative standard deviations of the concentrations less than 3.6%. Furthermore, by applying a short plug of low-pH buffer after injection, the analysis of greater volumes of the urine sample has been carried out, resulting in detecting more peaks.     

Effect of contact angle on capillary rise in annular meniscus

The shapes of axisymmetric annular menisci are calculated for a range of contact angles at the constraining cylindrical walls. From these shapes the capillary rise (or depression) Δh relative to an infinite external meniscus is obtained in terms of the radii of the inner and outer cylinders and physical properties of the liquids. This value of Δh may then be used to correct the measured rise in a concentric capillary tube relative to the annular meniscus. The capillary rise, Δh, has been experimentally measured for several liquids and their surface tensions directly calculated from these values. Excellent agreement with accepted values is obtained, even though the measured capillary rise is very small, thus confirming the accuracy of the theoretical calculations and experimental technique.     

Rate of capillary rise in porous media with nonuniform pores

An equation for the rate of capillary rise in porous media with nonuniform pores has been derived on the basis of a three-dimensional network model of pore structure, consisting of a repeating capillary element with step changes in its diameter. The measured rates of capillary rise of water in a number of sandstone samples have been accurately predicted by the model. The apparent capillary diameter for the rate of the capillary rise has been found to be several orders of magnitude smaller than the pore diameter corresponding to the peak of the mercury intrusion porosimetry curve. The permeability of the sandstone samples has been found to be approximately directly proportional to the apparent diameter of the rate of capillary rise. This approximate proportionality has been shown to follow from the capillary network model of the pore structure used in this work.     

The transition from inertial to viscous flow in capillary rise

We investigate the initial moments of capillary rise of liquids in a tube. In this period both inertia and viscous flow losses balance the pressure generated by the meniscus curvature (capillary pressure). It is known that the very first stage is purely dominated by inertial forces, where subsequently the influence of viscosity increases (visco-inertial flow). Finally the effect of inertia vanishes and the flow becomes purely viscous. In this study we derive the times and meniscus heights at which the transition between the time periods occur. This is done in an attempt to provide a method to determine a priori which terms of the momentum balance are relevant for a given problem. Analytic solutions known from previous literature are discussed and the time intervals of their validity compared. The predicted transition times and the calculated heights show good agreement with experimental results from literature. The results are also discussed in dimensionless form and the limitations of the calculations are pointed out.     

Bound water restrained by nanocellulose fibres

The higher-order structure of natural cellulose fibres changes in the presence of water. In order to investigate the effect of molecular level fibre structure, melting behaviour of water restrained by nano- and microcellulose fibre was measured by differential scanning calorimetry. Fibre size was measured by scanning electron microscopy and atomic force microscopy. It was found that the melting peak of water restrained by microcellulose fibre started at 250–260 K in a W _c (=mass of water/mass of dry sample) range from 0.5 to 1.2, whereas that of nanocellulose fibre was 230–237 K. Furthermore, peak temperature of melting of water restrained by nanocellulose was observed at around 270 K, in contrast, that of water restrained by microcellulose fibre was observed at ca. 275 K. Bound water content was calculated from melting enthalpy. Both non-freezing and freezing bound water of nanocellulose fibre was far larger than that of microcellulose. The above results suggest that a large amount of freezing bound water is restrained in nanocellulose fibres. It is thought that a larger number of isolated hydroxyl groups exist on the fibre surface.     

Separation of chitosan by degree of acetylation using simple free solution capillary electrophoresis

Chitosan is a biopolymer of increasing significance, as well as a renewable and sustainable material. Its main molecular characteristics are molar mass and degree of acetylation (composition). Precise average degrees of acetylation were measured by quantitative ¹H solution-state NMR spectroscopy. While number-average degrees of acetylation had already been determined by ¹H NMR spectroscopy, weight-average degrees of acetylation are also determined and may be more relevant for some properties, such as mechanical properties. We report the first separation of chitosan according to its degree of acetylation using free solution capillary electrophoresis. Capillary electrophoresis separates chitosan in the ‘critical conditions’: the molar mass plays little role and the separation is by the degree of acetylation. It characterises the heterogeneity of chitosan samples in terms of composition (dispersity of the distribution of degrees of acetylation). This heterogeneity (broad distribution of degrees of acetylation) cannot be neglected contrary to a common assumption found in the literature. This fast and easy separation will allow establishing a structure–property relationships.

Quantitative aspects of the separation of arsenical species by free solution capillary electrophoresis

Capillary zone electrophoresis has been used for the separation of arsenite, arsenate, monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and phenylarsonic acid. The separation was achieved using a fused silica capillary filled with a phosphate buffer. The on-column U.V. detector was operated at 190 nm. The influence of voltage and injected volume, reproducibility of calibration and recovery from a drinking water sample were studied. Using the calibration plot method and phenylarsonic acid as internal standard the mean recoveries for arsenate, MMA and DMA were 104%, 97% and 97% respectively.     

Possibility of different time scales in the capillary rise around a fiber

By molecular modeling simulations, we study the dynamics of the rise of a meniscus on the outside of a fiber. We develop methods to measure simultaneously the height of the liquid interface and the contact angle versus time. We observe that in the complete wetting case (with an equilibrium contact angle equal to zero), the dynamic contact angle theta(t) behaves asymptotically as t(-1) contrary to some experimental results where one observes t(-1/2) instead. Using the combined model describing the dynamics of wetting, we predict that there are two different time scale behaviors within this process related to the two dissipation channels: friction between the liquid and the solid, leading to t(-1), and hydrodynamics, leading to t(-1/2). Using this approach, we find that the maximal speed of spreading on a fiber is a nonmonotonic function of the equilibrium contact angle.     

距离约束分子动力学计算低聚核苷酸 d(GGTATACC)~2溶液中三维结构

8聚核苷酸 d(GGTATACC)~2溶液中的三维结构通过简单的高温和低温动力学计算以及能量优化的方法获得。计算中以标准B型双螺旋DNA为初始结构, 加上螺旋间的20个氢键约束和由核磁共振NOESY谱获得的186个NOE约束, 另外还加入二面角约束32个。计算所得的6个低能(-302±5kJ/mol)优势构象的均方根偏差(rmsd值)均小于0.06nm, 平均均方根偏差为0.043nm, 从优化结构反算所得的NOE谱与实验数据相吻合。计算结果表明, 所加各约束项合理, 优化结构具有较高的可信度。     

An analytic model for atomic clusters

The paper presents an analytic model which is based on approximation of the catchment basins in cluster potential energy surface by structures of ash-tray type. In order to see how the quantities to be specified relate to those obtained by means of numerical computation, predictions for the Ar₁₃ caloric curve are made and they are compared with computer simulation data.