Flow into capillary gaps

FLOW INTO CAPILLARY GAPS 10.1.1 Predicted penetration rates... 

The process depends on a liquid metal flowing over surfaces to form a fillet between components and into the gap between the components, and then solidifying to form a permanent bond. Thus it is essential that the braze experiences high temperature capillary attraction. Without such attraction, solid braze material placed between components will flow out of the gap, sweat , when it melts. Any residue of non-wetting liquid that remains within the gap will not conform to the microscopic features of the component surfaces but form an array of voids, as illustrated schematically in Figure 10.1, that is mechanically deleterious and should be avoided if at all possible. The size of such voids can be decreased if an external pressure is used to confine a non-wetting liquid braze into a gap but cannot eliminate them because the pressure needed to shrink voids increases as they become smaller. 

The Reynolds numbers for the flow of a molten metal along a capillary braze gap are usually less than 1000 as will be shown later, and the theoretical laminar flow rates for such configurations have been calculated by Milner (1958) for both horizontal and vertical joints. He regarded the flow into a horizontal joint induced by capillary attraction as being impeded only by viscous drag, and derived a simple parabolic expression to describe such behaviour,... 

The prime requirement for successful brazing is that capillarity causes the molten braze to flow into gaps between the components to be joined. Unless the capillary attraction is sufficient to achieve this, the joint cannot be formed. Having formed a joint, the next requirement of the ultimate user is usually that it should be robust (strong or tough and preferably both strong and tough). Whether this requirement can be met also depends substantially, but not exclusively, on capillary phenomena. 

This was solved by use of a jet separator where the column effluent was passed across a very narrow gap between two jets and the highly diffusable carrier gas was largely removed, whereas the heavier analyte molecules crossed the gap without being vented. The problem of removing the carrier gas no longer exists since GC capillary columns provide a flow rate of 0.5-2 ml/min, which can be directly introduced into the mass spectrometer without it losing vacuum. 

Macaulay measured the time of flow of a viscous liquid drawn by its own surface tension into the narrow gap between planes along a horizontal capillary, and up a vertical capillary. Menneret, and Subrahmanian and Venkataraman, used the damping of oscillations of a liquid in a U-tube. 

A liquid-junction interface has also been suggested and applied for CE-ESI-MS [8]. Electrical contact with this interface is established through the liquid reservoir which surrounds the junction of the separation capillary and a transfer capillary, as shown in Fig. lb. The gap between the two capillaries is approximately 10-20 fim, allowing sufficient makeup liquid from the reservoir to be drawn into the transfer capillary while avoiding analyte loss. The flow of makeup liquid into the transfer capillary is induced by a combination of gravity and the Venturi effect of the nebulizing gas at the capillary tip [8]. 

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