Micromechanics of Soft Particles Mingyu Guo, Hans M. Wyss* Introduction Many polymers, complex fluids, and biological materials contain soft, deformable particles as one of their main constituents. Examples are latex paints or pastes and creams used in the food and drug industry, which contain soft microgels as fillers, or blood, which contains deform- able and compressible cells. As opposed to suspensions and gels that consist of solid particles, the mechanical behavior of such materials is still surprisingly poorly understood. The macroscopic mechanical behavior of particle suspensions, gels, or glasses often changes qualitatively if soft, deform- able particles are used instead of hard particles. A good example for this difference in macroscopic properties is the concentration dependence of the steady shear viscosity. For suspensions of hard particles, a drastic increase in viscosity is observed as the concentration of particles approaches a volume fraction around f  60%, close to the volume fraction for random close packing f RCP of spheres. [1,2] As the particles are not compressible or deformable their packing density is restricted to a limiting concentration, often corresponding to random close pack- ing f RCP  64%. However, for soft microgel particles this limit no longer applies. Because the particles are highly compressible, they can shrink to accommodate more particles within the same volume. As a consequence, the viscosity of microgel suspensions depends much less sensitively on concentration than the viscosity of a suspension of hard particles. Thus, to achieve the same Full Paper M. Guo, H. M. Wyss Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, Eindhoven, The Netherlands Fax: þ31 40 244 7355; E-mail: H.M.Wyss@tue.nl M. Guo Laboratory of Chemical Biology, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands H. M. Wyss Department of Mechanical Engineering/Polymer Technology, Eindhoven University of Technology, Eindhoven, The Netherlands Materials that contain soft, deformable particles exhibit a rich range of macroscopic mech- anical properties. Experimental access to the mechanics at the scale of a single particle is the basis for studying and understanding the macroscopic mechanics of these materials. In this paper, we discuss experimental methods that can be used to characterize the mechanics of microscopic soft particles. We focus on the recently developed capillary micromechanics method, which yields the full linear elastic behavior of a single particle. We validate the method by comparing results for the com- pressive modulus to osmotic compression measurements, which provide the most direct and unambiguous measure of compressibility. We find good agreement between the two methods on a system of deformable and com- pressible poly-N-isopropylacrylamide micro- gel particles. Our results thus support the validity of the capillary micromechanics method and suggest that it could be applied to a wide range of materials that consist of deformable soft objects. Macromol. Mater. Eng. 2011, 296, 223–229 ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com DOI: 10.1002/mame.201000359 223