Capillarity

Abstract

One of the most common fluid mechanical effects exploited in microfluidics is capillarity, e.g., induced fluid motion in very small channels. As we have seen, curved surfaces introduce a pressure gradient that can be exploited in order to drive fluids. In Eq. 20.11, we can see that for small radii, this pressure drop can amount to significant values. This is exploited by using channels with very small diameters; in the simplest case a circular capillary is used (see Fig. 21.1a). From Eq. 20.11, we can deduce that for a circular tube for which r1 = r2 = r, the pressure difference is given as pinside − poutside = 2 γ r (Eq. 21.1)