Capillary liquid transport

Brakel, J., Pore space models for transport phenomena in porous media review and evaluation with special emphasis on capillary liquids transport. Powder Technology, 1975, 11, 205-236. 

As stated earlier, CEP and CC are the most common materials used in the PEM and direct liquid fuel cell due fo fheir nature, it is critical to understand how their porosity, pore size distribution, and capillary flow (and pressures) affecf fhe cell s overall performance. In addition to these properties, pressure drop measurements between the inlet and outlet streams of fuel cells are widely used as an indication of the liquid and gas transport within different diffusion layers. In fhis section, we will discuss the main methods used to measure and determine these properties that play such an important role in the improvement of bofh gas and liquid transport mechanisms. 

We can observe electro-osmosis directly with an optical microscope using liquids, which contain small, yet visible, particles as markers. Most measurements are made in capillaries. An electric field is tangentially applied and the quantity of liquid transported per unit time is measured (Fig. 5.13). Capillaries have typical diameters from 10 fim up to 1 mm. The diameter is thus much larger than the Debye length. Then the flow rate will change only close to a solid-liquid interface. Some Debye lengths away from the boundary, the flow rate is constant. Neglecting the thickness of the electric double layer, the liquid volume V transported per time is... 

In electroosmosis, the stationary and mobile phases are exchanged in relation to electrophoresis. As measurement of the rate of movement of a liquid through a capillary is difficult, the force that it exerts is measured, i.e. the electroosmotic pressure, or, alternatively, the volume of liquid transported through a capillary in a given time interval. The electroosmotic velocity, veo, is... 

The first results about foam electrokinetics have been reported by Sharovamikov [62,63]. An electroosmotic liquid transport is observed in foams from solutions of ionic surfactants (NaDoS, CTAB, PO-3A, etc.) and it is larger than in systems with solid capillaries (specific transport from 1.6-1 O 6 to 210 6 m3 C 1). The maximum electroosmotic pressure depends on the initial pressure in borders and reaches 1 Pa. The addition of dedecanol to the NaDoS solution sharply decreases the electroosmotic transport but increases the electroosmotic pressure. To reduce the influence of border and film non-homogeneity that originates in a static foam under gravity, the electrokinetic studies have been performed in an advancing foam [62]. The specific electroosmotic transport depends on the capillary pressure and reaches a maximum value at pg = 0.5 kPa. The streaming potential (up to 10 mV)... 

The driving force behind the liquid transport has been determined by Kramer and Bubeck to be capillary pressure [17]. The velocity of liquid flow V is expressed by Darcy s equation... 

Alternatively, instead of measuring the electroosmotic liquid flux at zero pressure difference across the capillary, we can determine by measuring the counterpressure, AP q, necessary to completely stop the net liquid transport through the capillary ... 

Diffusion—Dissolved materials retained in the capillary liquid and in the surface liquor are transported by the wash medium... 

The evaporation process is also influenced by the liquid transport process. When liquid water cannot diffuse into the fabric, it can only evaporate at the lower surface of the fabric. As the liquid diffuses into the fabric due to capillary action, evaporation can take place throughout the fabric[39]. 

The centrifugal microfluidic platform uses inertial and capillary forces on a rotating microstructured substrate for liquid actuation. Relevant inertial (pseudo-) forces include the centrifugal force, Euler force and Coriolis force. The substrate is often disk-shaped. Liquid flow is possible in two dimensions but with the limitation that active liquid transport is always directed radially outwards. Active components can be limited to one rotational axis. 

Liquid transport in paper is, as can be understood from the above examples, a complex issue. Imbibition can occur by various mechanisms and is affected by many factors, such as the structure of the fibre network which controls the size of the capillaries and continuity, fibre surface chemistry and morphology, sizing and other chemical treatments of the fibre surfaces. Each practical case must be carefully thought over in order to make fruitful predictions of the absorbency behaviour and a correct analysis of the experimental data. 

One of the earliest methods for determining the zeta potential was to measure the volumetric flow rate of a liquid transported in a capillary under electroosmotic flow. This method seems simple in principal however, the small flow rates typical of electroosmotic flow ( nl/min) make it difficult to obtain accurate measurements. Normally the flow rate is determined by following the... 

Respiration Joint lubrication Capillary phenomena in liquid transport Arteriosclerosis Blood transport Emulsification of nutrients Enzymes Cell membranes... 

Figure 10 Enrichment and separation setup for SLM extraction. A - peristaitic pump, B - membrane device with hoiiow fiber instaiied into fused siiica capiiiary, C - iniet on the donor side, D - washing iine used after enrichment, E - microinjection pump to transport the acceptor into ioop F, G - Cis packed capillary column, H - detector window, I - splitter unit, J - high pressure pump. (Adapted from Thordarson E, Palmarsdottir S, Mathiasson L, and Jonsson JA (1996) Sample preparation using a miniaturized supported liquid membrane device connected on-line to packed capillary liquid chromatography. Analytical Chemistry 68 2559-2563.)...

Due to the small dimensions of capillaries the flow resistance for pressurized flow of liquid increases. Thus, alternative methods for liquid transport are used and at present the most important principle is electroosmosis requiring the integration of electrodes, usually platinum electrodes, and the application of electric fields. Additionally, centrifugal forces are used for propelling liquid through capillary systems. In some systems these principles are combined with pressurized liquid transport. 

Movements of liquid water within porous solids have been experimentally observed. Haines studied the movement of liquid water within porous media during drying [26, 27]. He observed rapid movements of water elements in the porous system (Haines jumps). The capillary pressure transports water to narrow pores at the surface. Larger volumes surrounded by narrow pores can be rapidly emptied, and smaller volumes can be filled when the water is removed out of the narrow... 

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