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 80-130 mu m were compressed to a wide range of deformations at different speeds (10-1000 mu m s(-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 (500f.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 T-1/2. This new highspeed 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. (c) 2005 Elsevier Ltd. All rights reserved.