On the Relation between the Behaviour of Pendent Liquid Droplet and the Phenomenon of Metal Transfer in Welding

Abstract

For the measurement of surface tension by the conventional drop weight method, the wettability of the rod to a liquid drop is not taken into account. When, however, the contact angle is comparatively great and the rod is less wettable to the liquid the wettablity of the rod is naturally expected to have an influence on the falling phenomenon of the drop
When the soap bubble, whose contact angle is 90 degrees, is adhering to the apex of a cone, whose base angle is a, the adhesive force F of the bubble to the cone is expressed by
πdγcosα-1/4⋅πd²P=F (1)
where d; diameter of contact circle of soap bubble
γ; surface tension of soap bubble
P; internal pressure of soap bubble
Assuming P is constant, F and d can be expressed as a quadratic curve, with F attaining a maximum under the following condition,
F_max=πdγ/2⋅cosα (2)
When the soap bubble is exposed to a wind, the adhesive force F will increase against the wind pressure with a decrease of diameter d up to the above value ; but when d becomes smaller than the above value, F will begin to diminish until the bubble will be fast blown off from the apex of the cone. At F_max, one of the principal values of curvature near the contact line becomes zero.
If this is applied to a molten droplet adhering to a cone, whose base angle is β, with contact angle θ₀, the following equation can be obtained:
F_max=πdγ/2⋅sin(β+θ₀) (3)
The point of zero curvature in the profile of the droplet under gravity corresponds to an inflection point of the curve. In the case of a very small droplet adhering to a relatively big cone, the droplet grows up until the horizontal circle intersecting the inflection point coincides with the contact circle of the droplet, when gravity is equal to adhesive force of the droplet.
It was suggested that a possible change of the melting tip of the electrode from flat to conical profile might be an influencial factor in the transfer transition mechanism of droplet from drop to spray in MIG welding