Molecular dynamics simulations of capillary rise experiments in nanotubes coated with polymer brushes

The capillary filling of a nanotube coated with a polymer brush is studied by molecular dynamics simulations of a coarse-grained model, assuming various conditions for the fluid-wall and fluid-brush interactions. Whereas the fluid is modeled by simple point particles interacting with Lennard-Jones forces, the (end-grafted, fully flexible) polymers that form the brush coating are described by a standard bead-spring model. Our experiments reveal that capillary filling is observed even for walls that would not be wetted by the fluid, provided the polymer brush coating itself wets. Generally, it is found that the capillary rise always proceeds through a t1/2 law with time t while the underlying molecular mechanism differs for wettable and nonwettable walls. For wettable walls, fluid imbibition is compatible with the Lucas-Washburn mechanism whereby the total influx of matter drops steadily with growing chain length N and the meniscus speed goes through a minimum at intermediate chain lengths. Moreover, because of flow, the polymer brush reorganizes its structure by forming a dense plug of chain segments under the meniscus that follows the meniscus in its motion. When the tube wall does not wet, one observes no meniscus formation for short chains although the fluid seeps through the wet brush. For a brush coating with longer chains, axial segregation between the brush segments and the fluid occurs by a kind of diffusive spreading, reminiscent of invasion percolation transport in a random medium, leading to the formation of a moving meniscus. For even longer chains that reach the tube axis, the rise of a meniscus with vanishing curvature-like imbibition in a porous medium is observed to take place.     

Expressions for evaluating the possibility of slip at the liquid-solid interface in open tube capillary electrochromatography

In this work, expressions are constructed and solved that describe the velocity field of electroosmotic flow (EOF) in open tube capillary electrochromatography (CEC) systems when the possibility of having unequal tangential velocities at the liquid-solid interface is considered and a slip condition is employed as a boundary condition for the velocity of the EOF at the capillary wall. The coupled equations of hydrodynamics (momentum balance equation) and electrostatics (Poisson equation) are solved numerically in order to obtain the distribution of the velocity field as well as the value of the volumetric flow rate in the open tube. Also, expressions for the velocity field and the volumetric flow rate of the EOF are presented that are valid for certain electrolytic systems and for certain parameter values for which analytical solutions to the momentum balance and Poisson equations could be obtained. The results presented in this work indicate that having slip in the velocity of the EOF at the wall of the capillary could (i) substantially increase the electroosmotic velocity in the plug-flow region of the radial domain of the open capillary tube and (ii) increase the portion of the radial domain of the open capillary tube where the velocity of the EOF has a plug-flow profile, which in turn could increase the average velocity and volumetric flow rate of the EOF in the open capillary tube. Furthermore, the modeling approach and the results presented in this work indicate a method for experimentally evaluating the possibility of having slip in the velocity of the EOF at the capillary wall.     

Application of capillary gas chromatography for physico-chemical measurements particularly considering the influence of adsorption on the liquid phase-tube wall interface

Summary Utilization of capillary chromatography for the quantitative definition of the thermodynamic values of the solute/liquid phase interaction is shown. A method is given to define the pure values of the distribution coefficients of volatile compounds in a gas-liquid phase system with the use of an intermediate standard for which adsorption interaction can be neglected. It is shown that the definition of the heats of solution of polar solutes in a cross-linked liquid phase leads to higher values if adsorption interaction is not taken into account.     

Measurement of the surface tension of liquid marbles

The capillary rise and Wilhelmy plate methods have been used to study the "surface tension" of water marbles encapsulated with polytetrafluoroethylene (PTFE) powders of 1-, 35-, and 100-μm particle size. With the capillary rise technique, a glass capillary tube was inserted into a water marble to measure the capillary rise of the water. The Laplace pressure exerted by the water marble was directly measured by comparing the heights of the capillary rise from the marble and from a flat water surface in a beaker. An equation based on Marmur's model was proposed to calculate the water marble surface tension. This method does not require the water contact angle with the supporting solid surface to be considered; it is therefore a simple but efficient method for determining liquid marble surface tension. The Wilhelmy method was used to measure the surface tension of a flat water surface covered by PTFE powder. This method offers a new angle for investigating liquid marble shell properties. A discussion on the nature and the realistic magnitude of liquid marble surface tension is offered.     

In Process Citation

A rapid simple method is described for the determination of carbon in organic compounds. The technique is based on a flash-combustion of the weighed sample (1-3 mg) in a hot empty combustion chamber at 950-1050 degrees and swept by a stream of oxygen at a rate of 80-100 ml min . Together with this flash and dynamic combustion in oxygen, a cupric oxide filling at 850 degrees is necessary. Halogens and sulphur oxidation products are retained by means of a silvered alumina filling at 750-800 degrees . Water and acidic oxides of nitrogen are absorbed at room temperature by means of magnesium perchlorate and manganese dioxide respectively. Carbon dioxide is absorbed in the cathodic compartment of an automatic coulometer and generates H(+) ions which are neutralized by OH(-) obtained by electrolysis. This method makes possible the determination of carbon in compounds containing halogens (including fluorine), phosphorus and metals. However, some metal compounds with polynuclear rings in their molecules give low results and require the addition of an oxidant to the sample in the boat for complete combustion. Vanadium pentoxide seems to be the best oxidant in the present working conditions. Solid samples are weighed and analysed in platinum or porcelain boats; liquid samples are weighed in Pyrex capillaries which are laid in platinum boats and covered with a small piece of platinum gauze. When normal liquid samples are analysed, one tip of the capillary is broken before its introduction into the combustion tube but in the case of volatile liquid samples the sealed capillary is introduced into the combustion chamber, where it explodes. The precision obtained is better than that of the classical methods.     

Influence of cyclohexane vapor on stick-slip friction between mica surfaces

Stick-slip friction between mica surfaces under cyclohexane vapor has been investigated with the Surface Force Apparatus. The dynamic shear stress decreased from 60 to 10 MPa with increasing relative vapor pressure (rvp) from 5% to 50%. Between a rvp of 50% and 80%, the shear stress remained at approximately 10 MPa, with a slight decrease on increasing the rvp. At a rvp greater than 80%, the values of shear stress were below 5 MPa. The stick-slip behavior was observed in the rvp range of 20% to saturation. When the rvp reached 20%, stick-slip appeared but faded out with sliding time. At a rvp greater than 50%, the stick-slip pattern was stable without fading. By taking into account the size of the meniscus formed by capillary condensation of the liquid around the contact area and the Laplace pressure, the dependence of shear stress and the stick-slip modulation on rvp suggests that the origin of the stick-slip observed in cyclohexane vapor is as follows: At a rvp greater than 50%, where stable sick-slip is observed, the stick-slip caused by the cyclohexane layering in the contact area is of essentially the same origin as that observed with mica surfaces sliding in bulk cyclohexane liquid. As with the bulk liquid experiment, decreasing the layer thickness (or the number of the layers) between the surfaces increases the shear stress at the onset of slip. In the vapor phase experiments, the stick-slip is enhanced by the increase of the negative Laplace pressure in the capillary condensed liquid, thereby forcing the surfaces toward each other more strongly with decreasing rvp. In the rvp range between 20% and 50%, where the fading stick-slip is observed, the condensate liquid seeps into the contact area under the influence of the applied tangential force and thus triggers the slip motion. Due to the small condensation volume, the liquid condensed around the contact area is exhausted in the process of repeating stick-slip. As the slip length is limited to the meniscus size, the stick-slip amplitude becomes smaller, and eventually the surfaces start sliding without stick-slip.     

Effect of Pulsed Ultrasonic Field on the Filling of a Capillary with a Liquid

The filling of capillaries with liquids of various viscosity in the ultrasonic field is experimentally investigated. It is shown that the breakage of the meniscus and the fast intense dissolution of the gas that filled the capillary occur at the threshold switching-on of the ultrasonic field in low-viscosity liquids. This causes the residual (after switching-off of the ultrasound) filling of the capillaries with a low-viscosity liquid. It is established that the residual filling of capillaries is observed only with the pressure amplitudes P _m exceeding some critical value; the residual filling increases with P _m and disappears as cavitation is induced below the capillary end. It is shown that pulsed regime is optimal for the ultrasonication.     

Liquid flow inside a cylindrical capillary with walls covered with a porous layer (Gel)

Viscous incompressible liquid flow in a long cylindrical capillary, the internal surface of which is covered with a permeable porous layer, is studied within the frameworks of three mathematical models. In the first model, the liquid flow in the porous layer is described by the Brinkman equation; according to the second one, the presence of the porous layer is taken into account using the Navier slip boundary conditions; and, in the third model, the Navier condition is imposed on the porous layer-liquid interface, with the flow inside the porous layer being excluded. The theoretical predictions are compared with the experimental data that one of us has obtained for liquid flow rates in porous capillaries. The validity and appropriateness of the application of the proposed models are discussed.     

Rise of liquids and bubbles in angular capillary tubes

We discuss the rise of a liquid inside an angular capillary tube. It is shown that for a wetting liquid, the height of the rise is (as usually) inversely proportional to the length which characterizes the confinement. The exact laws deduced from energetic considerations are found to be in excellent agreement with the data. We then show how such tubes can be used to prevent bubbles from being trapped. The rising velocity of a bubble is finally discussed, in the particular case of a square tube.     

Effect of surface morphology on measurement and interpretation of boundary slip on superhydrophobic surfaces

Highly liquid repellent surfaces have been obtained by the combination of roughness and hydrophobicity. Studies have reported that the flow over such surfaces exhibits larger boundary slip as compared to the smooth hydrophobic surfaces. However, the surface roughness can also lead to apparent slip. Thus, the effect of the two factors, that is, wettability and roughness, needs to be segregated. In this study, we have measured the slippage of water on rough hydrophilic and hydrophobic surfaces using colloidal probe atomic force microscopy technique (CP‐AFM). Results showed that the effect of surface roughness on the measured slip is dominant over that of wettability. It was also found that slip on surfaces with sparsely distributed asperities is highly local and measurements on various locations give dissimilar results. The results suggested that the main reason of the larger slip, on rough hydrophobic surfaces, is likely to be the roughness and not the hydrophobicity. Moreover, it was also found that the slip does not vary considerably with the increase or decrease in the shear rate. Most likely, this kind of slip phenomena is caused by the apparent decrease of the drag force, because the nanoasperities on the surface restrict the probe from reaching the surface properly. Copyright © 2016 John Wiley & Sons, Ltd.     

Calculation of retention factors in capillary electrochromatography from chromatographic and electrophoretic data

Retention factors in capillary electrochromatography (CEC) were calculated by means of theoretically derived equations and experimentally determined parameters in microcolumn liquid chromatography and capillary zone electrophoresis. It was found that the retention factor of uncharged components in CEC was about 20% higher than was calculated. The derived equations do not take into account alteration of the nature of the stationary phase or distribution constant by the applied electric field. However, the influence of the electric field on the retention in CEC can be estimated. Individual field contributions could not be determined.     

Liquid slip on a nanostructured surface

We explored a liquid slip, referred to as the Navier slip, at liquid-solid interface. Such a slip is provoked by the physicochemical features of the liquid-solid system. The goal of this study was to investigate the effect of a nanoengineered surface structure on liquid slip by fabricating the self-assembly structure of nano Zinc oxide (n-ZnO). We have also examined how the liquid-solid surface interaction controlled by hydrophobic chemical treatment affects the liquid slip. The findings showed that liquid slip increases with decreasing the characteristic length scales (e.g., channel height and depth), resulting in drag reduction. It was also found that dewetted (Cassie) state due to the generation of air gap developed by n-ZnO was more critical for the liquid slip than the minimization of interface interaction. The linear and nonlinear Navier slip models showed that liquid slip behavior is more obvious when increasing the nonlinearity. This study will contribute to understanding of the underlying physics behind fluid slip phenomena, such as the Navier slip for Newtonian liquids and Maxwell's slip for Newtonian gases.     

毛细管电泳中的毛细管区段灌注及其相关技术

毛细管区段灌注是一种随手性毛细管电泳、亲和毛细管电泳和胶束电动力色谱-质谱发展而产生的实验技术。由于它能消除采用以上方法时遇到的缓冲溶液对检测的影响。因此引起人们的关注。本文对区段灌注技术的原理,区段灌注技术在手性毛细管电泳、亲和毛细管电泳和胶束电动力色谱中的应用,以及目前区段灌注技术存在的一些问题进行了评述。本文还对由区段灌注技术发展而来的逆流毛细管电泳、液相预柱毛细管电泳、多步配体进样等实验技术进行了简介。     

New high-performance cryofocalizer injector for in-tube solid-phase microextraction and headspace capillary gas chromatographic applications

The construction of a high-efficiency but cheap injector for volatile and very volatile compounds is shown. The device focuses the compounds in a fused-silica (FS) transfer capillary with the aid of liquid nitrogen. A 6.2 mm O.D. glass tube liner (ca. 25 cm x 1.5 mm I.D.) is inserted in the heated (-200 degrees C) injector of the gas chromatograph in place of the standard glass liner, and extends further externally through a liquid nitrogen container made with styrofoam-like material. Inside this glass tube, the FS transfer line passing through the oven door is connected like a pre-column to the analytical high-resolution GC column. It can move fast between the heated and the cooled zone (<-->, deltaL = 13 cm), and when this movement starts, crvofocused analytes are injected "at once" resulting in symmetrical and sharp injection bands with "zero" carryover. The performance of this device is demonstrated by its application to in-tube solid-phase microextraction and to spice volatiles analysis.     

A macrothermodynamic approach to the limit of reversible capillary condensation

The threshold of reversible capillary condensation is a well-defined thermodynamic property, as evidenced by corresponding states treatment of literature and experimental data on the lowest closure point of the hysteresis loop in capillary condensation-evaporation cycles for several adsorbates. The nonhysteretical filling of small mesopores presents the properties of a first-order phase transition, confirming that the limit of condensation reversibility does not coincide with the pore critical point. The enthalpy of reversible capillary condensation can be calculated by a Clausius-Clapeyron approach and is consistently larger than the condensation heat in unconfined conditions. Calorimetric data on the capillary condensation of tert-butyl alcohol in MCM-41 silica confirm a 20% increase of condensation heat in small mesopores. This enthalpic advantage makes easier the overcoming of the adhesion forces by the capillary forces and justifies the disappearing of the hysteresis loop.     

Dual Adsorber-Capillary Column System for Gas Chromatographic Analysis of Air Samples

Abstract

An interface which allows thermal desorption and subsequent capillary gas chromatographic analysis of air samples is described. A small solid-sorbent trap is positioned between the sampling tube and capillary column. A sample thermally released from the sampling tube is transferred by a carrier gas at high flow rate to the trap and retained. From there it is again thermally released and transferred to the capillary column by carrier gas at a low flow rate, as required by capillary GC. The transfer and injection steps are effected by means of externally placed solenoid valves. The performance of the system depends on the desorption temperature and time allowed for transfer of the sample between the two adsorbers and the column. These parameters are programmable and can be changed to suit the requirements of a particular analysis. The system allows the analysis of sub-parts-per-billion concentrations of organic compounds in a comparatively simple and reproducible manner. Operation of the system does not require cryogenic cooling of either the trap or the GC oven. Chromatograms of a variety of air samples are presented and discussed.