Tunable viscosity modification with diluted particles: when particles decrease the viscosity of complex fluids

Abstract

While spherical particles are the most studied viscosity modifiers, they are well known only to increase viscosities, in particular at low concentrations. Extended studies and theories on non-spherical particles find a more complicated behavior, but still a steady increase. Involving platelets in combination with complex fluids displays an even more complex scenario that we analyze experimentally and theoretically as a function of platelet diameter, to find the underlying concepts. Using a broad toolbox of different techniques we were able to decrease the viscosity of crude oils although solid particles were added. This apparent contradiction could lead to a wider range of applications.Introduction. The demand of viscosity modifications by solid particles has many applications in the field of food science [1] and oil recovery [2]. The simplest theoretical description was found by Einstein for diluted spherical particles [3]. At that point, the predicted changes of the viscosity were small, especially for large dilutions in the concentration range of 1%vol. However, applications usually manifold either at large concentrations (as in the case of chocolate [4]) or in asymmetric particles, such as clay, which has many uses [5]. Therefore, the theoretical concepts became more complicated [6,7], and the large number of possibilities opens up a wide range of inter-pretations.The medium in nearly all these cases has been assumed to be a simple liquid. However, the response of complex fluids [8,9] to particles [10,11] can be more complicated [12,13], and, therefore, back coupling might be possible. In our past studies we have studied the lubrication effect, which describes the lamellar ordering [14] of a complex fluid, i.e a microemulsion (µE) next to a hydrophilic wall. This ordering allows the domains for facilitated sliding along the surface [13]. In neutron scattering studies we have found that the typical relaxation times in these lamellae are also faster than in the bulk. So for platelet particles, a facilitated flow behavior (lower viscosity) might be expected [12]. The change of platelet diameters revealed that the perfection of lamellar modes is increasing with increasing platelet diameter. Consequently, large platelets would serve the aim of reduced viscosities better than small ones.Until now, the studied liquids consisted of components with highly similar viscosities. Therefore, the rearrangement of domain structures resulted in similar viscosities, and the highly desired case of viscosity reduction was not found.

Characterization of a Bicontinuous Microemulsion Flowing along a Planar Surface

“…Staying at low concentrations of ca. 1%vol, the clay particles lead to decreased macroscopic viscosities with increasing platelet diameter [5,6] in agreement with the lubrication effect. For this observation it was inherently assumed that the lamellar order is not destroyed by the flow-field.…”

Rheological and Technological Aspects in Designing the Properties of Shear Thickening Fluids

“…The scientific literature also provides information on the many modifiers of the rheological properties of STFs and other fluids used in industry. These modifiers can influence the viscosity in a significant way, even when used in small amounts [ 50 ]. The additives used include halloysite [ 51 ], aluminum oxide [ 52 ], cellulose nanofibers [ 53 ], silicon carbide [ 52 ], boron carbide [ 52 ], carbon nanotubes [ 18 ], graphene and graphene oxide [ 54 , 55 ], expanded graphite [ 56 , 57 ], or poly(propylene glycol) diacetate [ 58 ].…”

Heterogeneity-induced retraction in viscoelastic fluids following cessation of flow

“…The effective zero-shear viscosity and elastic modulus are harmonic means of the properties in each fluid region weighted by the fraction the region occupies in the gap:and τ eff = η 0,eff / G 0,eff is the effective relaxation time. This form of the effective viscosity has been proposed previously for perfectly lamellar multicomponent flows, 35,36 which are analogous to shear bands. In Section 4.4, eqn (10)–(12) are examined for cessation of flow.…”