  Materials with continuous dissipation can exhibit responses and
functionalities that are not possible in thermodynamic equilibrium. While this
concept is well-known, a major challenge has been the implementation: how to
rationally design materials with functional non-equilibrium states and quantify
the dissipation? Here we address these questions for the widely used colloidal
nanoparticles that convey several functionalities. We propose that useful
non-equilibrium states can be realised by creating and maintaining steady-state
nanoparticle concentration gradients by continuous injection and dissipation of
energy. We experimentally demonstrate this with superparamagnetic iron oxide
nanoparticles that in thermodynamic equilibrium form a homogeneous functional
fluid with a strong magnetic response (a ferrofluid). To create non-equilibrium
functionalities, we charge the nanoparticles with anionic charge control agents
to create electroferrofluids where nanoparticles act as charge carriers that
can be driven with electric fields and current to non-homogeneous dissipative
steady-states. The dissipative steady-states exhibit voltage-controlled
magnetic properties and emergent diffuse interfaces. The diffuse interfaces
respond strongly to external magnetic fields, leading to dissipative patterns
that are not possible in the equilibrium state. We identify the closest
non-dissipative analogues of these dissipative patterns, discuss the
differences, and highlight how pattern formation in electroferrofluids is
linked to dissipation that can be directly quantified. Beyond electrically
controlled ferrofluids and patterns, we foresee that the concept can be
generalized to other functional nanoparticles to create various scientifically
and technologically relevant non-equilibrium states with optical, electrical,
catalytic, and mechanical responses that are not possible in thermodynamic
equilibrium.
