Structuring of Nanoparticle Suspensions Confined Between Two Smooth Solid Surfaces
Combining colloidal-probe atomic force microscopy and small angle X-ray scattering, the characteristic lengths determining the structuring of nanoparticle suspensions confined between two smooth solid surfaces are analyzed. Monte Carlo simulations and int
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Structuring of Nanoparticle Suspensions Confined Between Two Smooth Solid Surfaces
4.1 Introduction Confining particles between two solid surfaces leads to damped oscillatory forces [1, 2]. This well-known effect is directly related to the oscillating particle density profile perpendicular to the surface [3, 4]. The oscillatory force occurs when the oscillating concentration profile of the particles in front of the opposing confining surfaces overlap. With decreasing separation between the two confining surfaces, the layers of particles are pressed out one after another, which leads to measurable alternating repulsion and attraction. The oscillatory force thus indicates the periodic layering of confined particles. The force can stabilize the colloidal systems, since it hampers drainage of the film [5, 6]. The oscillatory wavelength represents the distance between two adjacent layers of particles formed parallel to the confining surfaces. The decay length is a measure of how far particles correlate to obtain periodic oscillations. There exists presently several techniques such as the surface force apparatus, [4] total internal reflection microscopy, [1] optical tweezers, [7] thin film pressure balance, [8, 9] and colloidal probe atomic force microscopy [10] to measure the oscillatory forces. The first study of the ordering of colloidal particles can be traced back to the 1980s. Nikolov et al. found that thinning films of aqueous dispersions of polystyrene latex nanoparticles changed thickness with regular step-wise abrupt transitions by using reflected light microinterferometry [11]. These observations verified that the step-wise thinning or stratification of thin liquid films could be explained as a layerby-layer thinning of ordered structuring of colloidal particles formed inside the film. There are several other papers that have also shown that particles tend to form periodic ordering during the approach of confining surfaces by methods of thin film pressure balance [12–14] and total reflectometry [15, 16]. Recently, the structuring formation has been studied by the measurement of the oscillatory force of colloidal particles by Piech and Drelich et al. [17–21] with colloidal probe atomic force microscopy (CP-AFM), which was advantageous in measuring the complete oscillatory force curves for various systems [17–27]. Y. Zeng, Colloidal Dispersions Under Slit-Pore Confinement, Springer Theses, DOI: 10.1007/978-3-642-34991-1_4, © Springer-Verlag Berlin Heidelberg 2012
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4 Structuring of Nanoparticle Suspensions Confined
Among those mentioned studies, the oscillatory wavelength of colloidal particles was found to depend on the bulk particle volume fraction φ according to λ ∝ φ−1/3 at relatively low volume fraction [17, 18, 20, 21]. At sufficiently high volume fraction, the wavelength was found to be close to the effective particle diameter, 2(R + κ−1 ) [19]. However, a precise understanding of the characteristic lengths, that is the wavelength and decay length (correlation length) of the oscillations in rela
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