Thermo-Rheological Memory of $κ$-Carrageenan Fluid Gels Formed Under Flow

Abstract

Fluid gels are soft materials formed by shearing biopolymer solutions during the sol-gel transition. Their ability to yield and flow beyond a critical stress makes them attractive for designing versatile, biocompatible materials in food, health care and medical applications. Although it is well established that both microstructure and mechanical properties depend on the shear applied during gelation, a unified physical framework linking these features remains lacking. Here, using $κ$-carrageenan gels as a model system, we use a combination of rheology and confocal microscopy to tackle their shear-induced structuring in fluid gels. We identify a thermo-rheological memory in $κ$-carrageenan gels formed under flow and show that it arises from a competition between shear and interparticle adhesion, captured by an Adhesion number. The resulting microstructural evolution is reminiscent of the behavior of attractive particulate dispersions under simple shear flow, thereby bridging gels made of macromolecules and particulate gels. This framework provides a route to tune fluid gel properties without altering their composition.

Figures (4)

• Figure 1: Rheology of $\kappa$-carrageenan fluid gels with $C_w = 0.35~\%~(w/w)$ and $I = 40~\mathrm{mM}$. (a) Apparent viscosity $\eta$ as a function of temperature $T$ during cooling under constant imposed shear rate $\dot{\gamma}_0$. The color scale indicates the applied shear rate. Vertical dotted line indicates the sol-gel transition at T = $37^{\circ}$C. Inset: apparent viscosity measured at $70^{\circ}\mathrm{C}$ as a function of $\dot{\gamma}_0$. (b) Flow curves of fluid gels formed at different shear rates, showing shear stress $\sigma$ versus $\dot{\gamma}$. The solid line represents the best fit with the Herschel–Bulkley model. Inset: dynamic yield stress $\sigma_y$ versus shear rate during cooling $\dot{\gamma}_0$. (c) Viscoelastic spectra showing storage ($G^{\prime}$, filled symbols) and loss ($G^{\prime\prime}$, open symbols) moduli as a function of angular frequency $\omega$. Solid and dotted lines represent best fits with a fractional Kelvin–Voigt model. Inset: plateau modulus $G_0$ versus shear rate during cooling $\dot{\gamma}_0$.
• Figure 2: Nonlinear viscoelasticity of $\kappa$-carrageenan fluid gels with $C_w = 0.35~\%~(w/w)$ and $I = 40~\mathrm{mM}$ measured from strain amplitude sweeps. (a) Storage modulus $G^{\prime}$ (filled symbols) and loss modulus $G^{\prime\prime}$ (open symbols) as a function of shear strain $\gamma$. The color scale indicates the shear rate applied during cooling, as in Fig. \ref{['fig:shear']}. (b,c) $G^{\prime}$ and $G^{\prime\prime}$ normalized by their linear-regime values, $G^{\prime}_{\mathrm{LIN}}$ and $G^{\prime\prime}_{\mathrm{LIN}}$, as a function of $\gamma$. The solid black line shows the response of the gel formed under quiescent conditions ($\dot{\gamma}_0 = 0~\mathrm{s^{-1}}$).
• Figure 3: (a) Magnitude of the overshoot in $G^{\prime\prime}$ measured during strain amplitude sweeps as a function of the preshear rate $\dot{\gamma}_0$ applied during cooling. Marker shape indicates the KCl-adjusted ionic strength: $I = 20$ mM (circles), 30 mM (triangles), 40 mM (squares), and 50 mM (diamonds). (b) $G^{\prime\prime}$ overshoot magnitude as a function of the normalized preshear rate $\dot{\gamma}_0 / \dot{\gamma}^*$. Inset: characteristic shear rate $\dot{\gamma}^*$ versus ionic strength $I$. The red line represents the best power-law fit with an exponent of 2.5.
• Figure 4: Confocal microscopy images of $\kappa$-carrageenan fluid gels formed at different shear rates during cooling. (a) Representative images of gels formed at (from left to right) $\dot{\gamma}_0 = \dot{\gamma}^*/5$, $\dot{\gamma}^*$, and $5 \times \dot{\gamma}^*$. [Scale bar: $100~\mu\mathrm{m}$]. (b) Example of a 3D volume reconstruction for the gel formed at $\dot{\gamma}_0 = \dot{\gamma}^*$. (c) Volume-weighted average particle radius as a function of the rescaled shear rate $\dot{\gamma}_0 / \dot{\gamma}^*$. Inset: probability distribution function of particle size.