Effect of particle roughness on the rheology of suspensions of hollow glass microsphere particles

https://doi.org/10.1016/j.jnnfm.2020.104235Get rights and content

Highlights

  • Chemical etching was used to vary roughness of hollow glass microspheres (HGMS).

  • Rheology tests of silicone oil-HGMS suspensions of 30% and 40% volume fraction were conducted.

  • The viscoity was shown to increase with roughness for 40% volume fraction.

  • The friction coefficent (µ*) was shown to increase with volume fraction.

  • µ* has a power-law dependence on shear rate, and decreases with increasing it, with effect being weaker at the higher concentartion.

Abstract

The rheology of high volume fraction (30–40%) suspensions with hollow glass microspheres (radius r = 15 µm) in a matrix of silicone oil has been studied experimentally. The surface of the hollow glass microspheres was roughened by wet chemical processing, using hydrochloric (HCl) acid and also sodium hydroxide etching. We obtained particles of various roughnesses in the range of Ra = 62–655 nm. The effect of particle roughness on the overall rheology is investigated, demonstrating its clear impact. The effect depends on the volume fraction, the magnitude of roughness and shear rate. At a 30% volume fraction, increasing the roughness ratio (Ra/r) from 0.42% to 4.44% does not result in a measurable change in viscosity. At a higher volume fraction of 40%, even a modest increase of the roughness ratio to 1.5% causes a significant increase in the viscosity of 117%, 97%, 50% and 39%, respectively at shear rates of 0.47, 1, 10 and 100 s−1. Increasing the roughness to 4.44% results in an increase of 272%, 105%, 41%, 23% in viscosity over the same range of shear rates. In all cases, we show that the effect of roughness diminished by increasing the shear rate. Some estimates of the average interparticle coefficient of friction are given- showing roughness increases friction. The friction coefficient is shown to increase with the volume fraction. Friction coefficient shows a power-law dependence on shear rate and decreases with the shear rate. The rate of decrease with shear rate is smaller for higher volume fraction of 40%.

Introduction

The rheology of non-colloidal suspension systems where the hydrodynamic forces are much larger than Brownian forces is of great importance in many applications. For systems with volume fractions larger than φ = 0.25, the chance of particle-particle interactions increases significantly. In such situations, the surface roughness of the suspended particles may become an essential factor in the overall rheological behaviour of the systems. More can be found in a recent review [17] about various aspects of suspension rheology.

The effect of particle roughness for colloidal suspensions has been studied by a few groups. Castle et al. [3] have shown roughness increased viscosity of latex suspensions. Hsiao et al. [5] also studied the effect of roughness of 2 µm colloidal PMMA particles. Hsu et al. [7] have studied the effect of roughness on shear thickening in colloidal suspensions. During the fabrication process of PMMA colloidal particles, they varied cross-linking agent concentration to introduce heterogeneous roughness. However, the SEM images showed some deviation from a spherical shape. Previous work by Lootens et al. [11] has also studied the effect of roughness on colloidal silica particles of 1 µm diameter for volume fractions 41–48%.

On the other hand, the effect of roughness on non-colloidal particles, which is our concern here, has been studied by Tanner and Dai [18] and Moon et al. [14]. They have previously shown that roughening a system of polystyrene microparticles in a matrix of Newtonian silicone oil or non-Newtonian Boger fluid results in a significant increase of up to 35% in viscosity. They have also shown that the results agreed reasonably well with mathematical models which took into account interparticle friction. In that work, mechanical roughening was used as an easy and effective way to generate surface roughness on smooth microparticles. In this process, cross-linking grinding and milling are used, as the autogenous roughening by other microparticles causes particle damage on the micro or nanoscale. This method was used on 40–80 µm sized particles, and roughness ratios (the ratio of average roughness to sphere radius, Ra/r) of 0.15% to 5.3% were achieved by varying the amount of time the particles spent in the grinding machine (0 to 10 h respectively). Although this roughening process is simple, there is evidence of some change in the shape of the particles, whereas for a satisfactory analysis of the results we need to keep an approximately spherical shape of the particles. Tanner and Dai [18] had confidence that by keeping the roughness ratio around 5% this objective would be fulfilled. To make sure this deviation from a spherical shape in previous experiments does not have an impact on the results, we decided to repeat the experiments by using a different roughening method that would better preserve the spherical shape of the particles.

A review by Hsiao and Pradeep [6] offers various methods used for roughening colloidal and non-colloidal particles. For the work in this paper, the wet chemical process offers a suitable approach to roughening the surface of microparticles. This method has been used widely in the industry on glass particles, and in dentistry to etch enamel parts. Zogheib et al. [20] used hydrochloric (HCl) acid to change the surface roughness and flexural strength of a lithium disilicate glass-ceramic. It was found that etching increased surface roughness and this increase was more significant for longer etching durations. Jang et al. [8,9] investigated the effects of hydrochloric acid and sulfuric acid on the surface of glass slides showing the RMS roughness could be increased by almost a factor of 10 depending on the type of the acid, the duration and processing methods. Shellenberger and Logan [15] used wet chemical etching to change the surface roughness of soda-lime glass beads. They used both sodium hydroxide (NaOH) and HCl and showed the effectiveness of the method to alter the surface roughness. Different wet chemical etching methods such as phosphoric acid were used for roughening enamel [4] where the RMS roughness could be increased by an order of magnitude.

In this work, we study the effect of particles surface roughness in suspensions containing rigid hollow glass microspheres. We have also used a roughening method different from that used by Tanner and Dai [18]. Here wet chemical etching is used to affect only the surface of the microspheres, and there is no deformation or shape change in the spherical form of the particles. We have done the experiments for suspensions of hollow glass microspherical particles (HGMP) in a matrix of silicone oil of 1.1 Pa.s viscosity.

Section snippets

Wet chemical etching of glass particles

The particles used in this study were hollow glass microspheres (HGMS) made from Soda-Lime glass which were supplied by Cospheric LLC, CA, with nominal diameters between 27 µm and 32 µm, and a density of 0.51 g/cc, as shown in Fig. 1a. The wet chemical etching was used to roughen the HGMS, and then 30% and 40% volume fraction suspensions with silicone oil were prepared. Before etching, a quantity of HGMS was weighed and placed into a beaker and submerged in 37% Hydrochloric acid solution. The

Results and discussion

Silicone oil was used as a Newtonian matrix. The measured viscosity of the silicone oil was 1.10 PaS at 24 °C. This fluid has an almost constant viscosity independent of shear rate between 0.1 and 100 s−1. Fig. 2 shows the viscosity of the untreated HGMS suspension system at 30% volume fraction. The results here are the average of two separate measurements. The results for suspension made with HCl and NaOH treated particles are also shown for comparison. There was a slight decrease in viscosity

Discussion

The tests presented above show that increased roughness generally increases the relative viscosity above the value for the smoothest spheres. One can estimate the average friction coefficient (µ*) for any roughness by using the bootstrap correlation [19]. It was shown there that the ratio of the frictionless relative viscosity (ηr*) to the frictional relative viscosity was, approximately,ηr=ηr*{1-kμ*P/τ}where k = 1.75 and P/τ is the ratio of the interparticle pressure P to the shear stress; in

Conclusions

From the results obtained here, we can conclude that the roughness increases viscosity. Here the methodologies for roughening the surface of the particles were such that there was minimal effect on the overall spherical shape of the particles. The results at the lower volume fraction of 30% for roughness ratios of 1.5% and 4.44%% showed increased viscosity only at the lowest shear rate due to sedimentation. There was no measurable effect on viscosity for shear rates above 1.0 s−1. However, for

Declaration of Competing Interest

We as the authors of the manuscript entitled “Effect of Particle Roughness on the Rheology of Suspensions of Hollow Glass Microsphere Particles” submitted to Journal of Non-Newtonian Fluid Mechanics for publication hereby declare that this manuscript is an original work, and it has not been submitted to nor published in any other journal. We declare no conflict of interest.

Acknowledgment

We thank Mr Hesamodin Jami for help in obtaining the STM images.

References (20)

There are more references available in the full text version of this article.

Cited by (0)

View full text