Is there any known material of which to construct a capillary tube that would make it possible to compress the diameter of the capillary tube while maintaining the internal shape? Intent being that one could dynamically manipulate the height of the fluid in the capillary by compressing and subsequently decompressing the tube?
Perhaps you may consider a three-layer sandwich tube: the outer layer is a rigid tube, the medium layer is a thick-walled tube made of open-cell porous rubber, the inner layer is a thin stretchable capillary (say, made of rubber). The outer layer is bonded to the medium layer, and the inner layer is bonded to the medium layer. Changing the pressure of gas inside the porous rubber of the medium layer you can change the diameter of the inner layer.