  We assess experimentally the ability of a simple flow-based sorting device,
recently proposed numerically by [Zhu et al., Soft Matter, 2014, 10,
7705-7711], to separate capsules according to their stiffness. The device
consists of a single pillar with a half-cylinder cross-section which partially
obstructs a flow channel so that initially centred, propagating capsules deform
and circumvent the obstacle into an expanding channel (or diffuser). We perform
experiments with millimetric capsules of fixed size which indicate that the
deviation of the capsule in the diffuser varies monotonically with a capillary
number - the ratio of viscous to elastic stresses - where the elastic stresses
are measured independently to include the effects of pre-inflation, membrane
thickness and material properties. We find that soft capsules with resistance
to deformation differing by a factor of 1.5 can be reliably separated in the
diffuser but that experimental variability increases significantly with capsule
stiffness. We extend the study to populations of microcapsules with size
polydispersity. We find that the combined effects of increasing capsule
deformability and relative constriction of the device with increasing capsule
size enable the tuning of the imposed flow so that capsules can be separated
based on their shear modulus but irrespectively of their size.
