Spontaneous capillary flow

Method of generating drastic and abrupt discontinuities in wettability with materials of dissimilar chemical nature utilized in promoting spontaneous capillary flow. 

Results from the development of these parallel plate surface-directed microfluidic have resulted in optimal device geometry recommendations. The optimal aspect ratio, defined as width per height of the device, has been determined through free energy considerations [3]. To ensure spontaneous capillary flow, the aspect ratio must follow Eq. 11 which indicates that the height of the device b must always be less than the width of the device a, and the capacity to promote fluid flow is subject to the... 

As already established in previous sections, the surface-directed approach to microfluidic system fabrication produces autonomously functioning devices with little rigor associated regarding implementation. This is a direct implication of the fact that the driving force for flow exploits capillary interactions producing spontaneous capillary flow. However, an unwanted artifact resulting from the inherent simplicity of these devices is that flow modu-... 

Keywords Spontaneous capillary flow (SCF), Gibbs free energy,... 

By definition, a spontaneous capillary flow (SCF) occurs when a liquid volume is moved spontaneously by the effect of capillary forces—without the help of auxiliary devices such as pumps or syringes. Capillary systems can be either confined or open, i.e. the Uquid moves inside a closed channel or in a channel partially open to the air. On the other hand, composite channels—sometimes partly open or with apertures—are increasingly used, and spontaneous capillary flow is a convenient method to move liquids in such geometries. Some examples of SCF are shown in Figure 1.1. 

In this chapter, we first investigate the conditions for spontaneous capillary flow in open or confined microchannels, composite or not, and we show that a generalized Cassie angle governs the onset of SCF [6]. Then we present the dynamics of the capillary flow with a generalized Lucas-Washburn-Rideal expression for the flow velocity and travel distance [7-9]. Finally, we focus on the particular effect of precursor capillary filaments— sometimes called Concus-Finn filaments [10,11]—that sometimes exist in sharp corners, depending on the wettability of the walls. 

Wetting of Solid Walls and Spontaneous Capillary Flow 5... 

Figure 1.1 Different examples of spontaneous capillary flows (SCF) in open-surface microchannels (channels etched in sihcon and coated hy an SiO layer) (a) serial SCF (b) parallel SCF (c) parallel channels (d) winding channels crossing wells (e) filling of a cylindrical cavity (f) capillary filaments in a cylindrical well (g) capillary filaments in corners. Photographs by N. ViUard, D. Gosselin and J. Berthier (CEA-Leti).

Composite channels with walls of different nature, and sometimes with virtual walls, i.e. open boundaries, are increasingly used in modern biotechnology. In such designs, capillarity is used to move the liquid through the system. In particular, spontaneous capillary flow (SCF) is especially interesting since it does not require any pressure to move the liquid. 

In this section, the dynamics of spontaneous capillary flows in open or confined microchannels, composite or not, are investigated. 

In this section, the general expression for the determination of the velocities of spontaneous capillary flows in composite, confined microchannels of arbitrary shapes is presented. This expression generalizes the conventional Lucas-Washburn-Rideal model, which is valid for cylindrical channels. It will be shown that the use of an equivalent hydraulic diameter in the Lucas-Washburn-Rideal model introduces a bias when the shape of the channel cross section differs notably from a circle. 

So far there have been only a few publications on suspended microfluidics [28,45,46]. Below we examine the case of a spontaneous capillary flow in suspended microchannels with parallel, vertical walls. First, we theoretically derive the conditions for SCF onset using an approach based on the Gibbs free energy [4]. Next, we verify the onset of SCF using the Surface Evolver numerical program. The dynamics of the hquid motion is then analyzed, using analytical arguments based on a force balance between the... 

Relation (1.69) can be cast in the general form proposed by Rye et al. for spontaneous capillary flows in V-grooves [18] ... 

The general condition for spontaneous capillary flow onset has been presented in this chapter it shows that SCF occurs when both geometrical and wetting conditions are met. In the case of composite channels, capillary flow may take place even if some of the walls are not hydrophilic, under the conditions that the other walls coxmterbalance their hydrophobicity. In the... 

J. Berthier, K.A. Brakke and E. Berthier, A general condition for spontaneous capillary flow in uniform cross-section microchannels. Microfluid Nanofluid. 16, 779-785 (2014). 

M. Kitron-Belinkov, A. Marmur, T. Trabold and G.V. Dadheech, Groovy-drops Effect of groove curvature on spontaneous capillary flow, Langmuir 23, 8406-8410 (2007). 

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