High-speed compression of single alginate microspheres

A recently developed high strain-rate micro-compression tester was used for the mechanical characterization of alginate microspheres produced by emulsification/internal gelation. Single microparticles with diameters of 80–130μm were compressed to a wide range of deformations at different speeds (10–1000μms-1), and then released, or held at constant deformation to permit them to relax. The higher speeds allowed compressions with minimal time-dependent behaviour of the particles, whether due to viscoelasticity or water loss. During compressions the force imposed on the particles was also measured so that force–deformation curves could be generated and analysed. A high-speed camera (500 f.p.s.) was used to capture images during compression and the subsequent release. Image analysis showed that the particles recovered fully when the gross deformation was 50% of the initial diameter, or less. This was taken to be the elastic limit for the particles. A 30% deformation was therefore chosen for compression/hold experiments. The faster the compression speed, the higher the force at a given deformation, implying that there was time-dependent behaviour. However, there was a plateau in the (reduced) elastic modulus-compression speed curve, which indicated that above a certain compression speed, time-dependent behaviour during compression might be neglected. Using such high speeds, the elastic modulus of microspheres was shown to be 330±4 kPa at 2% w/v (initial) alginate concentration. The force relaxation behaviour of the microspheres was characterized using the half relaxation time T1/2. This new high-speed micro-compression tester, with complementary high-speed video, is a powerful tool for investigating the mechanical properties of alginate particles and similar hydrated materials at the microscale.