Micron-scale restructuring of gelling silica subjected to shear

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Abstract

Hypothesis/objective

We examine the time dependent viscometric behavior of a well-defined system of gelling colloidal silica and how this behavior may be understood from a simple theoretical model which incorporates the microstructure of the gel. The ultra-small angle neutron scattering (USANS) technique is used to interrogate structure during the gelation process.

Experiments

The investigations focused on a system where both particles and interactions are well-defined: 7 nm silica particle acid-treated aqueous solution subjected to a constant applied shear in Couette geometry. Ultra-small angle neutron scattering (USANS) time-dependent scattering intensities were measured at wave vectors, q, in the range, 1.0 × 10−3 ≤ q/nm ≤ 7.3 × 10−2 coupled with viscosity data recorded simultaneously. The interpretation of the USANS scattering data is reliant on an isotropic sample. This assumption has been investigated, over a limited range of scattering vectors, using more suitable small angle neutron scattering (SANS) instrumentation with a restricted q-range.

Findings

The first recorded direct kinetic measurements of the micron-scale structure in a gelling system. A critical micro-structural feature of the intensity-viscosity time behavior of a gelling colloid subjected to a shear is the cluster size. A viscosity/intensity coupling observed at the time of a viscosity maximum that corresponds to a time-dependent critical stress and speculated to be independent of the wave vector over a wide q-range.

Introduction

Theoretical studies on the rheometric and viscometric [1], [2] properties and behavior of a colloidal gel subjected to an applied shear have attracted attention for a long time [3] and continue to do so [4], [5], [6], [7], [8], [9], [10]. How, for example, an applied shear influences the network formation of the gel, which by its nature resists flow, is of particular interest as are the length-scales of the gelling particles and their spatial correlations. The academic approach to this problem is reinforced by the commercially relevant connections with, for example, the food [11], cosmetic [12], bio-medication and pharmaceutical [13] industries, and such diverse areas as smart materials [10], [14], enhanced oil extraction [15], and rocket propellants [16].

For many years, the structural perspective on gelling systems has been dominated by the techniques of light scattering [17], [18], [19], [20], [21], [22], [23] and small angle neutron scattering (SANS) [24], [25], [26], [27], [28], [29], supported by small angle x-rays (SAXS) [30], [31]. Overall, the literature on gelation is widely based and rich, yet publications on the evolution of structure during aggregation/gelation of a system subjected to an external shear are scarce [5], [8]. In fact, at this time, kinetic scattered data appears to be limited to a SANS/viscosity study on a solution of gelling colloidal silica [27], and parallel light scattering experiments carried out by Cao and co-workers [21] on a similar system.

There is a challenge to extend the shear - influenced kinetic structural studies from the nano to the micron scale, in particular because kinetic data to support the conclusions of theoretical and simulation studies are not available. The choice, however, of appropriate experimental techniques to meet this objective is limited mainly because a gelling solution can become turbid or opaque as the gelation proceeds. Hence, several techniques, such as dynamic or static light scattering, have to be ruled out due to the problems associated with multiple scattering [32]. An initial probe, however, of the micron structure of shear influenced gelled silica was published [33] using the ultra-small angle neutron scattering (USANS) procedure which avoids the issues of multiple scattering and interrogates the micron scale [34], [35], [36], [37]. A related ultra-small angle x-ray scattering (USAXS) investigation of gelled silica was reported by Hoekstra [20]. These papers, however, did not address the viscosity/intensity kinetic behavior of the gelling sample: for the USANS study, the sample was pre-sheared off-line; for the USAXS study, an emphasis was on the static structure as a function of temperature and the applied shear rate. The studies, nevertheless, demonstrated that USANS and USAXS were powerful tools yielding important information on the structure of aggregates.

Both USANS and USAXS spectrometers have advantages and disadvantages as possible instruments with which to measure the kinetic structure of a shear-influenced gel. The background, however, of using USANS to investigate gels is well-established. An update on the approach in [33] – combining off-line rheometry with USANS spectroscopy – has yielded, in particular, structural insight into storage and loss moduli data [38], [39], [40]. Further, of relevance here, USANS viscometric and scattering experiments with an on-beamline Couette cell [28], [41], [42] are now discussed [43], [44].

Despite this significant progress, direct measurements of the micron-scale restructuring responsible for the intensity-viscosity time behavior of a sheared gelling system are not in the literature. The objective of this work was to report an example. Colloidal silica was the sample chosen for study; a choice made because the important surface properties of colloidal silica are well-known [1], [2] and the inter-particle interactions are relatively simple and well-defined [4].

Section snippets

Rheometer

The USANS intensity measurements were supported by viscosity data initially taken with the rheometer positioned off-line, but later placed in situ within the neutron path. The rheometer is a commercial MCR-500 (Anton Paar GmbH, Graz, Austria)1 device with an added purpose-built quartz Couette shear cell consisting of two concentric quartz cylinders. The inner cylinder has an outer

Sample preparation

The gels were prepared from a stock aqueous suspension of commercial grade Ludox AS-30 colloidal silica at pH ≈ 9.6 (Supplied by W.R. Grace & Co., Columbia, USA). The spheres were designated by the manufacturer to have a nominal diameter σ = 7 nm. Polydispersity was estimated at 20%. The suspension had a 31 mass% SiO2 (corresponding to a silica volume fraction, ϕ = 21%) with a suspension density of 1.2 g cm−3. Gelation was introduced by lowering the pH of the stock solution to pH ≈ 8.

Kookaburra kinetic shear protocol

Kookaburra’s settings were adapted to get acceptable results from the shearing experiments in a reasonable run-time, see Supplementary Material, A. Four values of the wave vector, Table 1, were selected to trace the scattering curve at a given time. The average time-dependent intensity was recorded by repeated cycling.

The experiment was carried out in two segments. The first, designated Run I, recorded the intensity time-dependence over an arbitrary selected span from shortly after gel

Discussion and conclusions

The similarity between the curves of Fig. 5 is noticeable even though the sample under shear is liquid-like, but a solid after the shear is removed. Both plots indicate a characteristic length of about 3–6 μm, at q ∼ 2 * 10−3 nm−1 using q = 2π/ξ: an estimate consistent with observations published by many authors who investigated cluster growth in silica gels with electron microscopy [55]. Three q-ranges can be loosely identified; low (q/nm ≲ 1 × 10−3), intermediate (1.0 × 10−3 ≲ q/nm ≤ 7 × 10−2

Acknowledgments

This work benefited from the use of the SasView application, originally developed under NSF Award DMR-0520547. SasView also contains code developed with funding from the EU Horizon 2020 programme under the SINE2020 project Grant No 654000. We are grateful to Norman Booth from the Sample Environment team at the Australian Centre for Neutron Scattering for his help in implementing Couette flow on the Kookaburra instrument. We are also grateful to Antony Van Dyk of The Dow Chemical Company, PA,

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