Micromechanics of Soft Particles

Abstract

Materials that contain soft, deformable particles exhibit a rich range of macroscopic mechanical 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 compressive 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 compressible poly‐N‐isopropylacrylamide microgel 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. magnified image

Micromechanics of temperature sensitive microgels: dip in the Poisson ratio near the LCST

“…The particle volume was calculated from the positions of the 6 characteristic points by assuming that the particle is rotationally symmetric along the central axis of the capillary and consists of 3 sub-volumes: a frustum of a cone with circular cross-section at the center, and two sphere caps at the front and the rear of the particle. 11,29 In this simple description of the particle deformation, the parameters V, R band and L band are sufficient to describe the shape and volume change of the particle in order to analyze its elastic properties.…”

Capillary micromechanics for core–shell particles

“…In equilibrium, the microparticle is no longer moving and the externally applied stress must be fully balanced by this internal elastic stress. In the absence of static friction between the particle and the capillary wall, the external forces 60 acting on the particle can be derived based on the applied pressure difference and the tapering angle 25,26 (see details of the derivation in supplementary information). The effect of friction is neglected for hydrogel materials since they exhibit very low friction coefficients with a glass surface 48,25,26 .…”

“…In the absence of static friction between the particle and the capillary wall, the external forces 60 acting on the particle can be derived based on the applied pressure difference and the tapering angle 25,26 (see details of the derivation in supplementary information). The effect of friction is neglected for hydrogel materials since they exhibit very low friction coefficients with a glass surface 48,25,26 . Thus, by 65 quantifying the particle deformation as a result of the applied pressure difference, the elastic properties of a single microparticle can be measured.…”

“…Mechanical properties control the conditions under which the core-shell microcapsules are deformed elastically, plastically or even ruptured [15][16][17][18][19][20][21][22][23][24] . They 25 directly determine the volumetric and shape changes of the microcapsules under an applied external stress 25,26 , which for soft and biological materials can be surprisingly large. For instance, biological microcapsules such as red blood cells can pass through capillary vessels that are much smaller than their equilibrium 30 diameters, thereby circumventing the clogging of the blood vessels [27][28][29]16 .…”

“…Recently proposed methods circumvent this problem by deforming the entire soft objects, such as cells and droplets, in 75 microchannels to correlate their elasticity to velocity, and use this correlation for sorting cells/droplets/particles [42][43][44][45][46] 26 and atomic force microscopy 47 . Since the method is straightforward and can readily be integrated into microfluidic devices, it is an ideal tool for measuring mechanical properties at the single particle level.…”