Shear jamming and nonlinear rheology of chocolate suspensions

Abstract

We experimentally investigate the rheology of dark chocolate pastes in both industrially relevant pre-refined form and simplified model systems. Steady and oscillatory shear experiments reveal yielding, pronounced shear-thinning, and stress-dependent hysteresis governed by solid loading. Fitting the viscosity data with the Maron-Pierce model provides stress-dependent maximum flowable fractions $φ_{\rm{m}}(σ)$, defining yield loci in the $(φ, σ)$ plane. Their variation with stress quantifies the coupled roles of friction and adhesion in setting flow limits. Large-amplitude oscillatory shear tests characterize transitions from elastic to viscous behavior and identify distinct recovery pathways near jamming. Contact-stress decomposition separates hydrodynamic and frictional contributions, confirming that adhesive contact networks dominate stress transmission in pre-refined pastes. These results establish chocolate pastes as dense, adhesive suspensions whose flow is controlled by the interplay of friction and adhesion, offering quantitative benchmarks for constitutive modeling and linking chocolate processing to the broader physics of constraint rheology.

Figures (20)

• Figure 1: Electron microscopy images of (from left to right) sucrose particle (one large with a few fragments), cocoa solid (one large and many smaller cocoa particles), pre-refined dark chocolate paste (more irregular sugar crystals mixed with cocoa particles that get nearly as big as the sugar but are also much smaller), and the dark model paste (one large sugar crystal decorated in cocoa particles).
• Figure 2: Particle size distributions of materials used in this work: cocoa powder, sucrose, as well as model and pre-refined dark chocolate pastes.
• Figure 3: Pre-refined samples. Relative viscosity as a function of both volume fraction (bottom axis) and fat content (top axis) at $\sigma = 50\,\rm{Pa}$. The Maron-Pierce fit is shown as the solid curve, with the corresponding maximum volume fraction $\phi_{\rm{m}} = 0.675$, which corresponds to a maximum fat content $\rm{wt}_{\rm{m}} = 24\%$. The optimal fitting parameters are found to be $\alpha = 0.57$ and $\beta = 2.7$. Error bars denote standard deviation ($\pm\,\rm{std}(\eta^s)$) across independent loadings. Representative raw signals at are shown in \ref{['fig:reproducibility_PreRef']}.
• Figure 4: Model samples. Relative viscosity as a function of (top) volume fraction and (bottom) fat content at $\sigma = 50\,\rm{Pa}$. The Maron-Pierce fit curves are shown as solid lines in the top panel, while the dashed lines in the bottom panel are a visual guide. Estimated maximum volume fractions $\phi_{\rm{m}}$ are indicated in the legend. For the model dark paste, $\alpha = 0.24$ and $\beta = 2.9$; for the liquor, $\alpha = 0.58$ and $\beta = 2.7$. Error bars denote standard deviation ($\pm\,\rm{std}(\eta^s)$) across independent loadings.
• Figure 5: Shear-rate sweeps for pre-refined pastes. (top) viscosity, (center) shear stress, and (bottom) normal force vs. shear rate for different fat contents. Lines are a visual guide. Magenta stars ($\star$) in top and bottom panels indicate points at $\approx 50\,\rm{Pa}$ as a link to \ref{['fig:etaS_vs_phi_stress50Pa_PreRef', 'fig:etaS_vs_phi_stress50Pa_Model']}.