A model of hard spheres trapped inside a container of fluctuating shape is
proposed to describe colloidal particles in a vesicle or in an emulsion dro
plet. The container is assumed to be the convex hull of the particles and i
s described by an integral geometric approach including volume and surface
terms. In the limit of large volume coupling, the model reduces to the well
-known geometric problem of natural bin packing. Using computer simulations
and cell theory, we calculate equilibrium properties for various finite nu
mbers of confined particles in conformations ranging from clusters to plana
r and linear structures and identify transitions between these different co
nformations.