Difference between revisions of "Spreading of nanofluids on solids"

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==Reference==
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Entry by [[Pei Kun Richie Tay | Richie Tay]] for AP 225 Fall 2012
Wasan, D.T., Nikolov, A.D., Nature 423 (2003).
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==Keywords==
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== General ==
adhesion, spreading, disjoining pressure
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'''Authors''': Darsh Wasan, Alex Nikolov
  
==Summary==
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'''Publication''': [http://www.nature.com/nature/journal/v423/n6936/full/nature01591.html Wasan, D et al. Spreading of nanofluids on solids. ''Nature'', 423, 156-159 (8 May 2003)]
  
[[Image:Spreading_1.jpg |right| |300px| |thumb| Figure 1. a.  Diagram of experimental setup.  b.  Actual picture of particle structuring.  c.  In-layer particle structure inside wedge film.]]
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'''Keywords''': [[Disjoining pressure]], [[Thin film]], [http://en.wikipedia.org/wiki/Nanofluid Nanofluid]
  
[[Image:Spreading_2.jpg |right| |300px| |thumb| Figure 2. a.  Disjoining pressure versus film thickness.  b.  Spreading coefficient as a function of film thickness.]]
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== Introduction ==
  
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Nanofluids are dilute suspensions of nanoparticulates (e.g. micelles, globular proteins, metal particles). They are widely investigated for their enhanced thermophysical properties [1], but their unique spreading behavior also makes them potentially useful for soil remediation, lubrication, and oil recovery, among other applications [2]. Unlike simple liquids, whose spreading velocity scales inversely with viscosity, nanofluids show enhanced spreading as nanoparticle concentration (and hence liquid viscosity) is increased [3]. Theoretical calculations suggest that this phenomenon could arise from organization of the nanoparticles in confined spaces; in this paper, the authors sought experimental evidence of this ordering using video microscopy.
  
The authors of the paper study adhesion and spreading of suspensions of nanometer-size particles, or nanofluids.  When a gas bubble dispersed in an aqueous nanofluid approaches a smooth hydrophilic solid surface, a microscopic transition exists between the liquid film and the meniscus.  This transition region has a wedge-like profile, and the nanofluid film can change in steps inside this region.  The aim of the paper is to find how structural disjoining pressure affects the spreading of colloidal fluids on solid surfaces.
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== Results and Discussion ==
  
In the main experiment, a wedge-film was formed by blowing an air bubble (diameter 200um) against a smooth glass plate in a suspension of 1um-latex spheres. The volume fraction of latex spheres was 7%. It was found that the latex particles form a 2D colliod crystal at a thickness of the wedge film equal to twice the particle diameter, but the structure changes to a disordered structure when the film grows in excess of three particle diameters. Figure 1 shows the particle in-layer distribution function. This corresponds with the theoretical prediction of disjoining pressure shown in Figure 2. The peaks are due to when integral multiples of the diameter of particles can be accommodated by the wedge. The spreading coefficient is also shown in Figure 2, and it was found that the spreading coefficient increases with a decrease in film thickness. Thus, the authors found that the in-layer particle structuring can enhance the spreading of nanofluids on solids.
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[[Image:nanofluid1a.jpg|400px|thumb|right|'''Figure 1.''' (a) Experimental setup for looking at particle ordering in the wedge region. (b)
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Particle structuring in a wedge film. Latex particles had diameter 1μm, charge 0.8μC/cm<sup>2</sup>, and occupied 7 vol.%. Figure from Ref. [2]]]
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[[Image:nanofluid2.jpg|500px|thumb|right|'''Figure 2.''' ''(Left)'' In-layer particle structure inside the wedge film. ''(Middle)'' Theoretical disjoining pressure profile on the wedge walls as a function of film thickness ''r'' scaled by particle diameter ''d'' (= 8nm). ''(Right)'' Calculated spreading coefficient arising owing to particle disjoining pressure. Figures from Ref. [2]]]
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[[Image:nanofluid3.jpg|400px|thumb|right|'''Figure 3.''' (a) Photomicrograph of the differential interference patterns formed at the
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three-phase (solid–liquid–air) contact region of an oil drop placed on a glass surface. (b–e) Photomicrographs taken at (b) 30s; (c) 2 min;
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(d) 4 min; and (e) 6 min after addition of an aqueous micellar solution of SDS. Figure from Ref. [2]]]
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To examine particle structuring, the authors started with a nanofluid comprising 7 vol.% charged latex spheres in deionized water. They trapped a 200μm-diameter air bubble under a glass plate in the nanofluid, and looked at the wedge-like three-phase contact region using reflected-light digital video microscopy (Fig. 1a). They found that the particles formed a 2D colloidal crystal at a wedge-film thickness twice the particle diameter, but beyond three particle diameters, the particles became disordered. Figure 1b shows the particles in the wedge region.
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The observed particle distribution pattern (Fig. 2, left) corresponded to calculations of the disjoining pressure <math>\Pi</math> (Fig. 2, middle), as well as to computer simulations. The oscillatory pattern arose from changes in particle-particle mean interaction potential as crystallization occurred in the wedge films. Also shown in Figure 2 (right) is the spreading coefficient ''S'' (estimated using [[Image:eq1.jpg]] and theoretical calculations for <math>\Pi</math>), which increased as film thickness decreased. The sharp change in the slope for ''S'' occured at the wedge thickness where there was particle ordering, thus proving that this structuring was what enhanced the spreading of nanofluids on solids.
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As a demonstration of the utility of this atypical spreading, the authors then simulated an oily soil removal process using an oil droplet immersed in an aqueous suspension of SDS (0.4 vol.%, 10x the critical micelle concentration). With the aid of differential and common three-phase contact angle interferometry, they showed the sequential penetration of the nanofluid between the oil and glass surface, leading to the formation of small aqueous lenses (white spots encircled by dark fringes) and the eventual separation of the oil drop from the glass by a thick aqueous film with a dimple (Fig. 3). They hypothesized that, as in the earlier experiment, micelle diffusion into the wedge film was responsible for setting up a disjoining pressure gradient that facilitated spreading of the micellar solution across the solid surface.
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By adding an electrolyte (0.1M NaCl) to the nanofluid, detachment of the oil drop was prevented. This might appear counterintuitive, since the interfacial tension at the interface between the oil and the nanofluid decreased at a higher salt concentration, which caused the drop to shrink and should have enhanced the separation process. The authors believed that the high salt concentration resulted in a decrease in the effective micelle diameter (and hence micelle volume fraction) due to shrinkage of the electrical double layer around each micelle. This could have been sufficient to reduce the disjoining pressure gradient and the driving force for drop detachment.
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This article is interesting because it showed empirically that particle structuring near a three-phase contact region is able to set up a disjoining pressure gradient that influences spreading of a liquid in that region. This is unlike simple fluids, where the spreading coefficient ''S'' is independent of film thickness; in nanofluids, ''S'' depends on the thickness of the liquid film, which affects organization of nanoparticles in that film.
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== References ==
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[1] [http://www.hindawi.com/journals/jnm/2012/435873 A Review on Nanofluids: Preparation, Stability  Mechanisms, and Applications. ''J Nanomaterials'', Volume 2012 (2012), Article ID 435873]
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[2] [http://www.nature.com/nature/journal/v423/n6936/full/nature01591.html Wasan, D et al. Spreading of nanofluids on solids. ''Nature'', 423, 156-159 (8 May 2003)]
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[3] [http://www.sciencedirect.com/science/article/pii/S0001868607001868 Sefiane, K et al. Contact line motion and dynamic wetting of nanofluid solutions. ''Adv Colloid Interfacial Sci'', Vol 138, Issue 2 (19 May 2008), Pages 101–120]

Latest revision as of 02:45, 13 November 2012

Entry by Richie Tay for AP 225 Fall 2012

General

Authors: Darsh Wasan, Alex Nikolov

Publication: Wasan, D et al. Spreading of nanofluids on solids. Nature, 423, 156-159 (8 May 2003)

Keywords: Disjoining pressure, Thin film, Nanofluid

Introduction

Nanofluids are dilute suspensions of nanoparticulates (e.g. micelles, globular proteins, metal particles). They are widely investigated for their enhanced thermophysical properties [1], but their unique spreading behavior also makes them potentially useful for soil remediation, lubrication, and oil recovery, among other applications [2]. Unlike simple liquids, whose spreading velocity scales inversely with viscosity, nanofluids show enhanced spreading as nanoparticle concentration (and hence liquid viscosity) is increased [3]. Theoretical calculations suggest that this phenomenon could arise from organization of the nanoparticles in confined spaces; in this paper, the authors sought experimental evidence of this ordering using video microscopy.

Results and Discussion

Figure 1. (a) Experimental setup for looking at particle ordering in the wedge region. (b) Particle structuring in a wedge film. Latex particles had diameter 1μm, charge 0.8μC/cm2, and occupied 7 vol.%. Figure from Ref. [2]
Figure 2. (Left) In-layer particle structure inside the wedge film. (Middle) Theoretical disjoining pressure profile on the wedge walls as a function of film thickness r scaled by particle diameter d (= 8nm). (Right) Calculated spreading coefficient arising owing to particle disjoining pressure. Figures from Ref. [2]
Figure 3. (a) Photomicrograph of the differential interference patterns formed at the three-phase (solid–liquid–air) contact region of an oil drop placed on a glass surface. (b–e) Photomicrographs taken at (b) 30s; (c) 2 min; (d) 4 min; and (e) 6 min after addition of an aqueous micellar solution of SDS. Figure from Ref. [2]

To examine particle structuring, the authors started with a nanofluid comprising 7 vol.% charged latex spheres in deionized water. They trapped a 200μm-diameter air bubble under a glass plate in the nanofluid, and looked at the wedge-like three-phase contact region using reflected-light digital video microscopy (Fig. 1a). They found that the particles formed a 2D colloidal crystal at a wedge-film thickness twice the particle diameter, but beyond three particle diameters, the particles became disordered. Figure 1b shows the particles in the wedge region.

The observed particle distribution pattern (Fig. 2, left) corresponded to calculations of the disjoining pressure <math>\Pi</math> (Fig. 2, middle), as well as to computer simulations. The oscillatory pattern arose from changes in particle-particle mean interaction potential as crystallization occurred in the wedge films. Also shown in Figure 2 (right) is the spreading coefficient S (estimated using Eq1.jpg and theoretical calculations for <math>\Pi</math>), which increased as film thickness decreased. The sharp change in the slope for S occured at the wedge thickness where there was particle ordering, thus proving that this structuring was what enhanced the spreading of nanofluids on solids.

As a demonstration of the utility of this atypical spreading, the authors then simulated an oily soil removal process using an oil droplet immersed in an aqueous suspension of SDS (0.4 vol.%, 10x the critical micelle concentration). With the aid of differential and common three-phase contact angle interferometry, they showed the sequential penetration of the nanofluid between the oil and glass surface, leading to the formation of small aqueous lenses (white spots encircled by dark fringes) and the eventual separation of the oil drop from the glass by a thick aqueous film with a dimple (Fig. 3). They hypothesized that, as in the earlier experiment, micelle diffusion into the wedge film was responsible for setting up a disjoining pressure gradient that facilitated spreading of the micellar solution across the solid surface.

By adding an electrolyte (0.1M NaCl) to the nanofluid, detachment of the oil drop was prevented. This might appear counterintuitive, since the interfacial tension at the interface between the oil and the nanofluid decreased at a higher salt concentration, which caused the drop to shrink and should have enhanced the separation process. The authors believed that the high salt concentration resulted in a decrease in the effective micelle diameter (and hence micelle volume fraction) due to shrinkage of the electrical double layer around each micelle. This could have been sufficient to reduce the disjoining pressure gradient and the driving force for drop detachment.

This article is interesting because it showed empirically that particle structuring near a three-phase contact region is able to set up a disjoining pressure gradient that influences spreading of a liquid in that region. This is unlike simple fluids, where the spreading coefficient S is independent of film thickness; in nanofluids, S depends on the thickness of the liquid film, which affects organization of nanoparticles in that film.

References

[1] A Review on Nanofluids: Preparation, Stability Mechanisms, and Applications. J Nanomaterials, Volume 2012 (2012), Article ID 435873

[2] Wasan, D et al. Spreading of nanofluids on solids. Nature, 423, 156-159 (8 May 2003)

[3] Sefiane, K et al. Contact line motion and dynamic wetting of nanofluid solutions. Adv Colloid Interfacial Sci, Vol 138, Issue 2 (19 May 2008), Pages 101–120