Determination of the elastic properties of single microcapsules using micromanipulation and finite element modeling

Finite element modeling (FEM) has been used to simulate the compression of single elastic core-shell capsules between two parallel plates. FEM allows characterizing the compressions at deformations beyond the wall thickness, when both bending and stretching contribute to the force resisting compression. Due to the incorporation of bending effects, the force deformation profiles of capsules with the same elastic modulus E depend on the wall thickness to capsule radius ratio (h/r). A model is presented that enables the (i) calculation of h/r from the individual compression force profiles at fractional deformations lower than 0.1, thus applicable for brittle capsules and for elastic-plastic capsules and (ii) calculation of Eh by comparison with FEM data at that h/r. Thus, the model allows the determination of E from compression data alone, as the wall thickness is also determined. The compression of melamine-formaldehyde capsules with a hexyl salicylate core using a micromanipulation technique is given as an example of the application of the model. The estimated wall thickness value, found to be independent of the capsule size, is in excellent agreement with transmission electron microscopy measurements.