Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Interaction of Particles with Surfactant Thin Films: Implications for Dust Suppression Xiaolong Zhu,†,‡ Deming Wang,† and Vincent S. J. Craig*,‡ †
School of Safety Engineering, China University of Mining and Technology, Daxue Road No. 1, Xuzhou, Jiangsu 221116, China Department of Applied Mathematics, Research School of Physics, Australian National University, Mills Road, Acton, ACT 2601, Australia
‡
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S Supporting Information *
ABSTRACT: Understanding the interaction of particles with foams is important in antifoaming applications and dust suppression. In the former, the aim is for the particles to break the foam, whereas in the latter it is desirable that the stability of the foam is maintained or enhanced. The interaction of particles of different wettabilities with thin surfactant films is investigated with a Sheludko cell, enabling the thinning and rupture of the films to be studied in the presence and absence of a particle, using white-light interferometry. The films were prepared from the surfactant cetyltrimethylammonium bromide and a commercial dust suppression foaming agent. The film lifetimes are extended upon the addition of hydrophilic particles and reduced upon the addition of hydrophobic particles with advancing contact angles >90°. The Laplace pressure in the film surrounding a particle is calculated as a function of the contact angle and particle size, revealing that the meniscus surrounding hydrophilic particles has a positive Laplace pressure, which increases the lifetime of the film.
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explosions.7,9−12 However, the dust concentration at the longwall face (point of coal extraction) and heading face (point of extraction for road construction) in coal mining is often above the standard limit, because it is difficult to completely suppress dust during excavation.13 The use of foams has been demonstrated to provide improved dust suppression efficiency compared with water spraying in field studies, especially for respirable dust.14−18 The foam is generated by continuously mixing air, water, and foaming agents in a spray that is directed toward the dust source. The particular arrangement of the nozzles varies and is often designed to reduce the amount of respirable dust in a particular location occupied by workers, rather than completely remove all dust. Details of how the spraying nozzles may be arranged are given in the work by Lu et al.18 Ideally, the foam, which occupies a large volume and possesses desirable rheological properties, envelops the dust source and captures the dust particles.13,19 The use of foams has the added benefit of reducing water consumption. Although the dust suppression efficiency can exceed 85%, the concentration of dust not captured is often still above safe levels.18 A better understanding of the fundamentals of dust particle interactions with foams is key to producing foams with greater dust suppression performance. The interaction between single particles and foams has been studied in the field of antifoaming.20−23 The influence of particles on foam stability was found to be strongly affected by
INTRODUCTION Exposure to respirable dust is a major health hazard causing considerable death and disability worldwide.1 In the 1930s, silicosis as a result of dust inhalation caused the deaths of at least 476 workers in the Hawk’s Nest Tunnel disaster.2,3 This remains the greatest industrial accident in the history of the United States. Today, the same health threats accompany activities such as drilling, cutting, grinding, and blasting. Exposure continues to threaten the health of workers as evidenced in a recent audit of workers in Queensland, Australia, which has already identified 98 new cases of silicosis, which has no known cure, among workers in a variety of industries. 4 Despite growing awareness and improved preventative technologies, deaths caused by dust inhalation (pneumoconiosis) grew from 251 000 deaths worldwide in 1990 to 260 000 in 2013.5 Improved technologies for the mitigation of dust exposure are required. Dust suppression is particularly important in the mining industry. The deaths of coal workers attributed to pneumoconiosis (CWP) in 2013 was 25 000, down from 29 000 in 1990,5 although the actual figures may be much higher due to under-reporting. Even in countries with the most advanced industrial mining processes, such as the United States, Australia, and Britain, newly detected cases of CWP have been reported in recent years,6 highlighting the need to improve methods of dust control. Many coal miners are also exposed to silica dust and are therefore also at risk of silicosis.7,8 In the mining industry, numerous technologies such as water spraying, water infusion, and ventilation methods are applied to reduce the quantity of airborne dust and the risk of dust © XXXX American Chemical Society
Received: December 20, 2018 Revised: April 18, 2019 Published: May 22, 2019 A
DOI: 10.1021/acs.langmuir.8b04230 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir the contact angle between the particle and the foaming solution.21,24 In coal mining, both hydrophobic particles (coal dust) and hydrophilic particles (silica dust) need to be controlled to protect the health of workers. Frye and Berg20 have proposed a model by which the contact angle determines the effect of particles on thin-film drainage and stability. In this model, a contact angle of greater than 90° is required for a spherical particle to promote film rupture. Here, we investigate the interaction of model particles with thin films of a surfactant in which the thickness of the surfactant film can be measured by interferometry, with the aim of understanding how the contact angle of the particle influences the lifetime of the film and the mechanism by which the film ruptures. Further, we aim to determine if this is consistent with the model for the antifoaming action of particles described by Frye and Berg20 and Aveyard and Clint.25 The particles used are spherical glass beads that in some cases have been treated to alter the surface properties to modify the wettability.
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R=
MATERIALS AND METHODS
Figure 1. Diagram of the Scheludko cell used to analyze film thicknesses and the influence of particles on foam stability. The internal diameter of the film holder was 4 mm. The particles, which were attached to a fine glass capillary, were introduced to the film manually using a micromanipulator.
liquid film was formed by injecting the surfactant solution into the film holder through a capillary. A microscope was used to observe and record the magnified image of the film. This provided a coaxial light path, which was perpendicular to the film and enabled color videos of the film to be recorded at a frame rate of 1 fps. Monochromatic intensities were obtained from the color images using ImageJ software (National Institute of Health, USA). The film thickness was determined in the usual manner28−30 by analyzing the monochromatic intensity of the reflected light that results from the interference of the reflected light from the front and back of the liquid film. The film thickness was calculated using eqs 1−3.27
Δ=
λ arcsin 2πz 1+ I − Imin Imax − Imin
Δ
(
4R (1 − R )2
)
(1 − Δ)
(3)
where n is the refractive index of the liquid, I is the measured intensity, and Imin and Imax correspond to the last intensity minimum and maximum. λ is the wavelength of light and h is the film thickness. The refractive index of the surfactant solution was obtained by adjusting the refractive index of water using a dn/dC value of 0.15 cm3 g−1.31 Red (λ = 635), green (λ = 525), and blue (λ = 460) software filters were used to process the images. The calculated film thicknesses were found to be independent of the filter selection. The cationic surfactant, cetyltrimethylammonium bromide (CTAB, Aldrich), and a commercial foaming agent used for dust suppression (DSFA, Anyun Mining Technology) were used to stabilize the liquid films. DSFA consists of 20−25 wt % primary alcohol ethoxylate, 9−14 wt % sodium alcohol ether sulfate, 5−8 wt % alkyl polyglycoside, 1−2 wt % sodium dioctyl sulfosuccinate, with the remaining 51−65 wt % being water. Thus, the DSFA contains both ionic and nonionic surfactants as shown in the Supporting Information, Table S1.1. For the purpose of calculation, we have assumed that the water weight fraction was 57.5%; this is an estimate based on Karl Fischer measurements allowing for the side reactions with surfactants in the DSFA. AR-grade ethanol was further purified by slow distillation using a spinning band distillation system (36−100 B/R Instrument Corporation). All water used was purified using a MilliQ gradient water purification system (Millipore). High-purity nitrogen gas was obtained from the boil-off of a liquid N2 storage facility. The sample particles used were glass ballotini spheres. The particle diameters were between 36 and 87 μm and were measured using an optical microscope. Although dust particles this large are produced, this is significantly larger than the size of inhalable dust particles, which are generally
