  We present a new system based on alginate gels for the encapsulation of a
ferrofluid drop, which allows us to create millimeter-sized elastic capsules
that are highly deformable by inhomogeneous magnetic fields. We use a
combination of experimental and theoretical work in order to characterize and
quantify the deformation behavior of these ferrofluid-filled capsules. We
introduce a novel method for the direct encapsulation of unpolar liquids by
sodium alginate. The addition of polar alcohol molecules allows us to
encapsulate a ferrofluid as a single phase. This encapsulation method increases
the amount of encapsulated magnetic nanoparticles resulting in high
deformations and offers possible applications of capsules as actuators,
switches, or valves in confined spaces like microfluidic devices. We determine
both elastic moduli of the capsule shell, Young's modulus and Poisson's ratio,
by employing two independent mechanical methods, spinning capsule measurements
and capsule compression between parallel plates. We then show that the observed
magnetic deformation can be fully understood from magnetic forces exerted by
the ferrofluid on the capsule shell if the magnetic field distribution and
magnetization properties of the ferrofluid are known. Using an iterative
solution scheme that couples a finite element / boundary element method for the
magnetic field calculation to the solution of the elastic shape equations, we
achieve quantitative agreement between theory and experiment for deformed
capsule shapes using the Young modulus from mechanical characterization and the
surface Poisson ratio as a fit parameter. This detailed analysis confirms the
results from mechanical characterization that the surface Poisson ratio of the
alginate shell is close to unity, that is, deformations of the alginate shell
are almost area conserving.
