Capillary fill time

When the capillary is installed, it is useful to determine the capillary fill time. This information will be useful for determining the length of time for each rinse step (NaOH, separation buffer or rinse solution), as well as for diagnosing potential capillary problem, such as partial or complete obstruction. One approach for accomplishing this (with ultraviolet, UV detection) is to pressure rinse the capillary with water, zero the detector, and follow with a rise with 100 mM, NaOH. The time required for a maximum change in absorbance to occur is the fill time to the detector. Factoring in the length between the detector and the capillary outlet yields the capillary fill time. ... 

Determine the capillary fill time. First fill the capillary with some low-absorbing solution (buffer or dH20) and then zero the detector. Using the rinse mode, fill the capillary with a strongly absorbing solution (e.g., 0.1 M NaOH) noting the amount of time to reach... 

FIGURE 9.4 Quick ampoule sampling of volatiles. Ninety-five percent ampoule filling time as a function of capillary diameter for 3 compounds. Calculation for filling through consecutive Knudsen diffusion into a vacuum, super sonic flow, and laminar flow. 

Inking Time — Pins may have different capillary fill and wetting rates depending upon surface characteristics and geometries. For example, inking times on quills are in the range of a few seconds, while it may not be necessary to keep solid pins in contact with a source plate for more than a second to allow uptake. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Fundamental knowledge about the behavior of charged surfaces comes from experiments with mercury. How can an electrocapillarity curve of mercury be measured A usual arrangement, the so-called dropping mercury electrode, is shown in Fig. 5.2 [70], A capillary filled with mercury and a counter electrode are placed into an electrolyte solution. A voltage is applied between both. The surface tension of mercury is determined by the maximum bubble pressure method. Mercury is thereby pressed into the electrolyte solution under constant pressure P. The number of drops per unit time is measured as a function of the applied voltage. 

Capillary electrochromatography (CEC) — A special case of capillary liquid chromatography, in which the mobile phase motion is driven by -> electroosmotic volume flow through a capillary, filled, packed, or coated with a stationary phase, (which may be assisted by pressure). The retention time is determined by a combination of -> electrophoretic mobility and chromatographic retention. 

In a qualitative sense, the measured position of the meniscus as a function of time was found to qualitatively follow the Washburn model (refer to the entry on Surface-Tension-Driven Flow, for details of this model). Quantitatively, however, a lowering of capillary filling speed could be noted, which might be attributed to the electroviscous effects and stronger surface influences over nanoscopic length scales. Future efforts need to be directed to develop more rigorous mathematical models to predict the quantitative trends of capillary fiUing in nanofluidic channels to resolve these issues. 

A maximum occurs in the characteristic displacement curves only when the surface tension relaxation is not as fast as compared to the capillary filling. More simplistic scaling estimates in the capillary rise phenomenon can readily be obtained by noting that within certain limits, the capillary rise represents a quasi-steady process, in which the amount of surfactant adsorbed to the solid/liquid interface per unit time is equal to that transported to the liquid/vapor interface by diffusion, which implies... 

To study the effect of surfactants, rates of evaporation of a nonionic surfactant solution from single quartz capillaries with radii from 10 to 20 pm were measured [15]. The results obtained for evaporation of pure water (curve 1) and 0.25% solution of syntamide-5 (curve 2) from capillaries of equal radii, r = 8.2 pm, are shown in Fig. 12. Here L is the distance of evaporating meniscus from the open capillary end and t is the time. The capillaries filled with water and with surfactant solutions, respectively, were placed in an evacuated chamber [16] near each other. Curves 1 and 2 refer to evaporation in vacuum (p/ps = 0) at A = 5 x 10 " cm /s, where K is the coefficient characterizing the rate of evaporation. The coefficient K = L j4t depends on external conditions of evaporation and is proportional to the difference between vapor pressure over meniscus, p , and in surrounding media, po-At first, the curves 1 and 2 practically coincide, but later on evaporation from the capillary filled with the surfactant solution gradually slows down. This can be explained by the concentration of surfactant molecules near the evaporating meniscus surface. 

As the liquid for filling all pores in LEPP we have used the fluorinated hydrocarbon Porofil 3 (Beckman-Coulter, USA). According to the manufacturer creos 0= 0.16 bar m (where cr is the surface tension, and 6 the contact angle). From comparison of capillary outflow-time for water and Porofil its viscosity was estimated as // = 1.25 mPa s. 

Figure 11.12 shows an example of the measured position of the moving meniscus versus time in capillary filling of hydrophilic nanochannels by deionized (DI) water (thin "native" silicon dioxide channel bottom and borosilicate glass channel roof, rectangular... 

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