Using capillary forces to control suspension rheology: Network structure, formation, and aging in capillary suspensions
Final Report Abstract
We investigated the structure of capillary suspensions in this project using a combination of imaging and rheological techniques. The rheological properties of a particle suspension can be substantially altered by adding a small amount of a secondary fluid that is immiscible with the bulk phase. The drastic change in the strength of these capillary suspensions arises due to the capillary forces, induced by the added liquid, leading to a percolating particle network. Using rheological scaling models, fractal dimensions are deduced from the yield stress and from oscillatory strain amplitude sweep data as function of the solid volume fraction. Exponents obtained using aluminum- oxide-based capillary suspensions, with a preferentially wetting secondary fluid, indicate an increase in the particle gel’s fractal dimension with increasing particle size. This may be explained by a corresponding relative reduction in the capillary force compared to other forces. Confocal images using a glass model system show the microstructure to consist of compact particle flocs interconnected by a sparse backbone. Thus, using the rheological models, two different fractal dimensionalities are distinguished: a lower network backbone dimension (퐷 = 1.86– 2.05) and an intrafloc dimension (퐷 = 2.57– 2.74). The latter is higher due to the higher local solid volume fraction inside of the flocs compared to the sparse backbone. Both of these dimensions are compared with values obtained by analysis of spatial particle positions from threedimensional confocal microscopy images, where dimensions between 2.43 and 2.63 are computed, lying between the two dimension ranges obtained from rheology. The fractal dimensions determined via this method corroborate the increase in structural compactness with increasing particle size. Capillary suspensions are three-phase fluids comprising a solid and two immiscible, liquid phases with unique texture and flow properties. The addition of a second, immiscible fluid affects structure and flow of suspensions including anisotropic particles was investigated. For needle-shaped and scalenohedral particles no increase in yield stress 휎y or storage modulus 퐺’ characteristic for a strong capillary force controlled, percolating particle network is observed when a secondary fluid is added. In contrast, a pronounced increase in 휎y and 퐺’ is found when a secondary fluid is introduced to suspensions of plate-like particles and optical as well as electron microscopy confirm the formation of a sample-spanning network characteristic for capillary suspensions. Suspensions of isometric particles exhibit a distinct maximum in 휎y or 퐺’ at low fractions of secondary fluid to particle volume fraction 휙sec ⁄휙solid ≈ 0.1– 0.2, whereas suspensions of plate-like particles exhibit constant 휎y and 퐺’ values over a wide range of 휙sec ⁄휙solid values up to ≈ 1 until spherical agglomeration occurs. Due to the different shape of the capillary bridges suspensions of plate-like particles can accommodate much larger fractions of secondary fluid until spherical agglomeration sets in than systems including spherical particles thus offering a versatile basic concept for the design of complex multi-component paste-like products.
Publications
- (2016). Influence of particle shape on the rheological behavior of three-phase non-Brownian suspensions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 497, 316-326
Maurath J., Bitsch B., Schwegler Y., Willenbacher N.
(See online at https://doi.org/10.1016/j.colsurfa.2016.03.006) - (2018). Fractal approaches to characterize the structure of capillary suspensions using rheology and confocal microscopy. Journal of Rheology, 62(1), 183-196
Bossler F., Maurath J., Dyhr K., Willenbacher N., Koos, E.
(See online at https://doi.org/10.1122/1.4997889)
