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The equation is derived for capillary flow in a cylindrical tube in the absence of a gravitational field, but is sufficiently accurate in many cases when the capillary force is still significantly greater than the gravitational force. In his paper from 1921 Washburn applies Poiseuille's Law for fluid motion in a circular tube.
An interesting phenomena, capillary rise of water (as pictured to the right) provides a good example of how these properties come together to drive flow through a capillary tube and how these properties are measured in a system. There are two general equations that describe the force up and force down relationship of two fluids in equilibrium.
Capillary action of water (polar) compared to mercury (non-polar), in each case with respect to a polar surface such as glass (≡Si–OH). Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of external forces like gravity.
The Starling principle holds that fluid movement across a semi-permeable blood vessel such as a capillary or small venule is determined by the hydrostatic pressures and colloid osmotic pressures (oncotic pressure) on either side of a semipermeable barrier that sieves the filtrate, retarding larger molecules such as proteins from leaving the blood stream.
The microcirculation has three major components: pre-capillary, capillary, and post-capillary. In the pre-capillary sector, arterioles, and precapillary sphincters participate. Their function is to regulate blood flow before it enters the capillaries and venules by the contraction and relaxation of the smooth muscle found on their walls.
If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables. [citation needed] According to Starling's equation, the movement of fluid depends on six variables: Capillary hydrostatic pressure (P c)
The interfacial (surface) tension, St, (dyne cm −1), can be calculated by applying the equation of capillary rise method (when the contact angle Ө → 0): = where: h (cm) is the height of Hg column above the Hg meniscus in the capillary; r (cm) is the radius of capillary
Jurin's law, or capillary rise, is the simplest analysis of capillary action—the induced motion of liquids in small channels [1] —and states that the maximum height of a liquid in a capillary tube is inversely proportional to the tube's diameter.