Ferrofluids!

from wikipedia:

A ferrofluid, (portmanteau of the Latin word ferrum, meaning iron, and the word fluid) is a liquid which becomes strongly polarised in the presence of a magnetic field.

Ferrofluids are colloidal mixtures composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrierfluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals forces and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale) paramagnetism, and are often described as "superparamagnetic" due to their largemagnetic susceptibility. Permanently magnetized fluids are difficult to create at present.^[1]

The difference between ferrofluids and magnetorheological fluids (MR fluids) is the size of the particles. The particles in a ferrofluid primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions. MR fluid particles primarily consist of micrometre-scale particles which are too heavy for Brownian motion to keep them suspended, and thus will settle over time due to the inherent density difference between the particle and its carrier fluid. These two fluids have very different applications as a result.

Sachiko Kodama's "Morpho Tower":
"This work is conceptualized as an "organic tower." The magnetic fluid, with a smooth black surface that seems to draw you in, reaches the top of the tower, spreading like a fractal, defying gravity.
The form and texture of the physical surface of the tower constantly alter between the hard iron core and the changing form of soft fluid. This is a dynamic sculpture, with reactable fluid surface"