Tunable shear thickening, aging, and rejuvenation in suspensions of shape-memory-endowed liquid crystalline particles

Abstract

The morphological features of particles, notably shape anisotropy, critically influence the rheological properties of dense suspensions, spanning both natural and engineered systems. This work explores the potential of using shape memory particles to dynamically regulate suspension fluid flow through controllable shape transformations. First, we synthesize shape-memory particles with programmable anisotropy from liquid crystal elastomers, such that the stiffness and shapes of the particles can be tuned by manipulating temperature. Our findings reveal that suspensions from such particles exhibit significant tunability in shear thickening behavior, transitioning from discontinuous shear thickening to a Newtonian-like response within a narrow temperature range of 60 [Formula: see text]C. This capability to modulate rheological responses in situ presents an approach for addressing processing challenges in many applications where control over flow behavior is paramount. Furthermore, we also show that suspensions composed of these anisotropic particles can undergo physical aging, and evolve into a glassy state. This state can be escaped upon activation of the shape memory effect. This reversibility underscores the potential for using such materials to engineer systems that can enter or come out of kinetic arrest by leveraging internal mechanical responses to external stimuli. The insights gained here not only broaden our understanding of the interplay between particle geometry and suspension dynamics but also pave the way for leveraging ensembles of stimuli-responsive objects to precisely control collective behaviors in many-body systems.

Keywords: colloids; dense suspensions; jamming; liquid crystal; stimuli-responsive.

Fig. 1.

Schematic for regulating suspension rheology with particle shapes. (A) Two types of shape anisotropy are illustrated. (B) Jamming state diagrams of a typical anisotropic and isotropic particle suspension are contrasted. (C) Illustration of the design of shape memory particle suspension in this work.

Fig. 2.

Synthesis, programmability, anisotropy, and shape memory of LCE microparticles. (A) Structures and the ratio of monomers for synthesis. Reactive groups are highlighted in red. The mesogenic moiety is highlighted in purple. (B) Schematic of vacuum drying process and microstructural changes in LCEs. (C) Bright field and polarized optical microscopy (POM) images of LCE microparticles, showing decreasing anisotropy and birefringence as the crosslinking density increases. (D) Controlling the particle shape and thermal response by varying the drying process. (E) Scanning electron microscopy (SEM) images and bright field images indicate that bulk vacuum-drying yields anisotropic LCE microparticles with a potato shape, whereas pea-like particles are obtained via freeze-drying in water. (F) DSC curves of normalized heat flow as a function of temperature for 30% crosslinked particles suspended in PEG200 show $T_{\mathrm{g}}$ around $-$7 ^°C and $T_{\mathrm{NI}}$ around 45 ^°C. (G) Shape memory response of the x-30 particles suspended in PEG200. Temperature increase above $T_{\mathrm{NI}}$ disrupts the nematic ordering and triggers a change to a spherical shape with reversibility upon cooling below $T_{\mathrm{NI}}$.

Fig. 3.

Mechanical properties of LCE x-30 film and steady-state rheometry data of freshly prepared suspension at 55 vol%. (A) Shear rheology dynamic temperature sweeps on solid LCE film after soaking in PEG200 for 2 wk. (temperature ramp rate $=$ 3 ^°C/min, frequency $=$ 1 Hz, and parallel plate geometry) showing storage moduli (blue line), loss moduli (green line) and tan delta (red line) of the LCE (See SI Appendix, Fig. S8 for the data of dry film). (B and C) Steady shear viscosity of LCE microparticle suspensions ($\varphi$ $=$ 55%) normalized with solvent viscosity shows temperature-dependent strength of shear thickening. (C) The data in (B) are replotted as a function of shear stress. A reference slope of 1 that corresponds to the onset of DST is shown with a black triangle in (C). (D) The strength of shear thickening ($\beta$) as a function of temperature for $\varphi$ $=$ 55% LCE suspensions decreases with temperature spanning from DST to CST values. Potato-like particles exhibit a remarkably higher $\beta$ at low temperatures compared to pea-like shapes. (E) A comparison of typical $\beta$ as a function of volume fraction from literature with values shown in (C) reflects the high tunable strength of shear thickening LCE suspensions, which in regular systems can only be achieved by increasing particle volume fraction. Data for silica particle suspension are extracted from ref. .

Fig. 4.

Physical aging and rejuvenation of LCE microparticle suspensions. (A) LCE microparticle suspension images reveal a solid-like behavior after some time under quiescent conditions. (B) Oscillatory shear rheology for the aged sample. The gray arrow corresponds to data acquired on increasing or decreasing shear stress ($\tau$). The red double arrow indicates hysteresis in $G^{\prime }$. (C) Steady shear viscosity as a function of shear stress for aged samples shows signatures of yielding except for the flow curve at 50 ^°C. (D) The aged LCE suspension flows when the sample is heated above $T_{\mathrm{NI}}$ and cooled back. (E) Oscillatory shear rheology for the rejuvenated sample. (F) Flow curves of LCE microparticles after heating to 55 ^°C and cooled back show good agreement with flow curves for fresh samples shown in Fig. 3C.

Fig. 5.

Mechanism of microstructure reconfiguration. (A) OM images captured on heating and cooling of aged suspensions shows the reconfiguration of the microstructure near $T_{\text{NI}}$. (Scale bar: 40 $\mathrm{\mu }\text{m}$.) (B) Schematics of the potential energy landscapes and the corresponding microstructures.