Suspesion rheology, yield stress, viscosity, capillary forces
Final Report Abstract
In this study, we investigated how the addition of a second immiscible fluid to a suspension can dramatically alter the rheological properties of the mixture from a fluid-like to a gel-like state or from a weak to a strong gel. The yield stress and viscosity increase by several orders of magnitude as the volume fraction of the second fluid increases and this transition is attributed to the capillary forces of the two fluids on the solid particles. This effect exists both when the secondary fluid preferentially wets the particles (pendular state) and even when the secondary fluid does not preferentially wet the particles (capillary state). In this study, we were able to determine that this is a basic physical phenomenon and such capillary suspensions can be created for a wide variety of liquid/solid combinations covering a range of particle sizes, contact angles, and liquid combinations. Each specific combination formed either a strong pendular state admixture or a strong capillary state admixture. These admixtures were characterized using the yield stress, viscosity, and shear modulus. We also investigated the parameters controlling the strength of these admixtures and their network properties. As this is a capillary force phenomenon, the parameters affecting the strength of the capillary force – the particle size and interfacial tension – will directly modify the rheological properties. The yield stress was found to be proportional to the interfacial tension and inversely proportional to the particles size, as predicted by theory. The volume fraction of solids also effects the rheological properties, but when the yield stress is normalized by the value without any secondary fluid, these values collapse onto a single master curve. Additionally, capillary suspensions are shear thinning such that at a sufficiently high shear rates there is no effect of secondary fluid content. We also investigated the microstructure and network properties of these admixtures and were able to show that small particle number clusters are stable and are likely the basis for the sample-spanning network in the capillary state. Finally, we investigated how these admixtures could be used to create smart, tunable fluids, stabilize mixtures that would otherwise phase separate, and the strong particle network can be used as a precursor for the manufacturing of cost-efficient porous ceramics and foams with unprecedented properties. These admixtures can be tuned through changes to temperature and the addition of surfactants. The strong capillary force induced particle network can also be used to create ceramic materials with high porosities (> 60%) at small pore sizes (1-10 µm). This process route is versatile and can be completed at significant cost and energy savings. Capillary suspensions can also be used to make thin films without or reduced content of stabilizers and rheology control agents that remain in the final dry film inhibiting the electrical properties thereby making these materials both less expensive and more efficient. These presence of the secondary fluid also inhibits crack formation in the drying film reducing the need for additional annealing steps. We also demonstrated that the strong particle network can possibly be used to to create ultra low-fat foods. While these various applications were only explored briefly in this current proposal and additional research will be required to bring such ideas to market, the present research was able to demonstrate the significant potential for capillary suspensions in these areas.
Publications
- Capillary forces in suspension rheology. Science, 301(6019) 897–900 (2011)
E. Koos and N. Willenbacher
- Capillary forces in suspensions: Rheological properties and potential applications. Chemie Ingenieur Technik, 83(8) 1305–1309 (2011)
E. Koos, J. Dittmann and N. Willenbacher
- Verfahren zur Herstellung einer porösen Keramik und eines porösen polymeren Werkstoffes sowie damit erhältliche Keramik und Werkstoffe. German Patent DE 10 2011 106 834 B3, dated 07.07.2011
J. Dittmann, E. Koos, N. Willenbacher and B. Hochstein
- Particle configuration and gelation in capillary suspensions. Soft Matter, 8(14) 3988–3994
E. Koos and N. Willenbacher
(See online at https://doi.org/10.1039/c2sm07347a) - Tuning suspension rheology using capillary forces. Soft Matter, 8(24), 6620–6628 (2012)
E. Koos, J. Johannsmeier, L. Schwebler and N. Willenbacher
(See online at https://doi.org/10.1039/c2sm25681a) - Ceramic capillary suspensions: Novel processing route for macroporous ceramic materials. Journal of the American Ceramic Society
J. Dittmann, E. Koos, Norbert Willenbacher
(See online at https://doi.org/10.1111/jace.12126)
