Dramatic effect of fluid chemistry on cornstarch suspensions: Linking particle interactions to macroscopic rheology

Suspensions of cornstarch in water exhibit strong dynamic shear thickening. We show that partly replacing water with ethanol strongly alters the suspension rheology. We perform steady and nonsteady rheology measurements combined with atomic force microscopy to investigate the role of fluid chemistry on the macroscopic rheology of the suspensions and its link with the interactions between cornstarch grains. Upon increasing the ethanol content, the suspension goes through a yield-stress fluid state and ultimately becomes a shear-thinning fluid. On the cornstarch grain scale, atomic force microscopy measurements reveal the presence of polymers on the cornstarch surface, which exhibit a cosolvency effect. At intermediate ethanol content, a maximum of polymer solubility induces high microscopic adhesion which we relate to the macroscopic yield stress.

Figure 1

(a) Experimental setup schematic. (b)–(d) Typical time evolution of vertical position $z$, velocity $\dot{z}$, and acceleration $\ddot{z}$ of the sphere for ${\mathrm{\Phi }}_{\text{EtOH}}^m=10\%$ and $H_{\text{fall}}=15$ cm. The insets in (c) and (d) show two zoomed-in oscillations with their respective mean velocities (${\overline{v}}_1,{\overline{v}}_2$) and mean accelerations (${\overline{a}}_1,{\overline{a}}_2$).

Figure 2

Sphere velocity $\dot{z}$ as a function of time for various ${\mathrm{\Phi }}_{\text{EtOH}}^m$ and $H_{\text{fall}}=10$ cm.

Figure 3

Non-steady-state and classical rheology as a function of ${\mathrm{\Phi }}_{\text{EtOH}}^m$: (a)–(c) Sphere velocity $\dot{z}$ as a function of time for various $H_{\text{fall}}$. (d)–(f) Drag force $F_D$ encountered by the sphere as a function of its velocity for the same and additional experiments. The gray squares in (d) show the mean drag force as a function of mean velocity $\overline{v}$ during oscillations. (g)–(i) Flow curves from classical rheological measurements (blue circles) and apparent flow curves obtained from settling experiments (orange diamonds). The dashed lines in (g)–(i) are the best fits of the data with the Bingham model and the orange line in (g) corresponds to the bulk oscillations' mean behavior.

Figure 4

Particle-particle interaction properties as a function of ${\mathrm{\Phi }}_{\text{EtOH}}^m$: (a) Typical force curve in water. The inset and arrows show the presence of pulling events (PEs) while retracting (red curve). The black bar corresponds to a vertical displacement of 30 nm. (b) Percentage of force curves exhibiting PEs. (c) Apparent Young's modulus $E^{*}$ of an individual CS grain and interaction length $L_{\text{int}}$ between CS grains. (d) Adherence force $F_{\text{adh}}$ between two CS grains for a contact time of 0 s (blue) and in the limit of infinite contact time (orange). (e) Yield stress measured from classical rheology (orange) and settling experiments (blue). The dashed lines are just guides to the eye. The color scales on the horizontal axis stand for the three typical behaviors: shear thickening and bulk oscillation (blue), yield-stress fluid (green), and shear-thinning fluid (yellow).

Figure 5

Scanning electron microscopy (SEM) image of (a) a cornstarch grain glued to the end of a tipless AFM cantilever and (b) a sample surface covered with cornstarch grains.

Figure 6

(a) Variation of ${\tilde{F}}_{\text{adh}}$ as a function of waiting time. Colors stand for each probing position. Dashed lines correspond to the exponential fit for each position and the solid line is the average fit. (b) Average ${\tilde{F}}_{\text{adh}}^{\infty }$ and $\tau$ as a function of ${\mathrm{\Phi }}_{\text{EtOH}}^m$.