Why does surface tension limit the height of a molten zone in float zoning?

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zone melting Figure 1: Schematic representation of (A) normal freezing, (B) zone refining

In the process of float zoning, a vertical rod of material is held by end clamps while a molten zone is moved along its length. Because the liquid zone is not contained by a vessel, it must be held in place by its own surface tension.

Surface tension acts as the primary force maintaining the stability of the liquid bridge between the two solid portions of the rod. This physical property theoretically limits the stable height of the molten zone; if the zone becomes too tall, the surface tension is insufficient to counteract the forces—such as gravity—that would cause the liquid to collapse or bulge, thereby destabilizing the process. To overcome this limitation and allow for the processing of larger or more stable zones, engineers have developed various induction heating procedures designed to stabilize the molten region at greater heights.1