Local structure and elasticity of soft gelatin gels studied with atomic force microscopy

Atomic force microscopy (AFM) measurements were done on aqueous gelatin gels submerged in dodecane. Use of home-built AFM instrumentation coupled with dedicated analysis of the recorded force-displacement curves, allowed the a posteriori extraction of both surface topography and elastic properties of these soft samples. Hertz theory was used to obtain (apparent) relative Young moduli (E*) from the force-indentation curves. For indentations smaller than 100 nm, scattered values of E* were found. This is partially attributed to the structural inhomogeneity of the polymer network at these length scales. At larger indentations, the relative Young moduli were found to be "compression-rate"-independent but to decrease with the indentation depth (delta). This independence of the compression rate indicates quasi equilibrium elastic behavior (i.e., the absence of stress relaxation by the gelatin), confirmed also by additional experiments, in which a truncated sawtooth driving voltage was used. In these latter AFM recordings, the compression regime is combined with a stationary piezo state when the AFM tip is in contact with the gel. The measured cantilever deflection due to gel relaxation was always below 10% from the indentation depth. Additional features were observed in the 3D recordings and associated with stiff fibrils lying on top of the soft gelatin network. Depending on aging time and location along the sample surface, mobile single fibrils as well as tough, compact, immobile fibril bundles were observed. A comparison was made between the relative Young moduli measured with AFM and the elastic (storage) moduli as measured in a conventional rheometer. Taking as variable the sample's "age", the microscopic and the macroscopic moduli turned out to be in good agreement in the limit of high (polymer) concentrated gels. Gel syneresis, with water exudation from the 10% gelatin network, was found to drastically increase the E* values found with AFM (at delta = 250 nm). The fact that this was not found with conventional rheometry might suggest a different syneresis behavior in dodecane.