Sorting of capsules according to their stiffness: from principle to application

TL;DR

This study experimentally demonstrates a flow-based device capable of sorting capsules by stiffness, showing reliable separation for certain stiffness differences and tunability for size and deformability variations.

Contribution

The paper provides the first experimental validation of a simple flow device for separating capsules based on their elastic properties, extending prior numerical work.

Findings

Capsules with 1.5x stiffness difference can be reliably separated.

Separation depends on capsule deformability and device constriction.

Flow tuning enables size-independent stiffness sorting.

Abstract

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.