Modeling of surface-tension-driven flow of blood in capillary tubes.

Surface-tension-driven blood flow into a capillary tube, as in some medical devices, is studied. In a previous article, we considered the early stages of the entry flow from a drop of blood into a capillary, and solved the problem analytically under the assumption that the resistance of the air is negligible. In the present note we consider a capillary tube of finite length, with the far end containing a small window which opens to the atmosphere. The dynamic reverberation of the air in the capillary tube is analyzed in conjunction with the dynamics of the blood. Existing computing programs are used to solve the Navier-Stokes equations. The interface is characterized by the surface tension between the blood and the air, and the contact angle at the triple point where the air-blood interface meets the capillary tube wall. The results tell us how good our earlier simplified analysis is. The new numerical results show that the smaller the window, the larger is the effect of aerodynamic reverberation. However, even for a window as small as 4% of the capillary cross section, and located at the end of the capillary, the difference of the time of arrival of the interface at the window is less than 5%.