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Measurement and Analysis of Forces in Bubble and Droplet Systems Using AFM ⇑ Rico F

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Measurement and Analysis of Forces in Bubble and Droplet Systems Using AFM ⇑ Rico F Journal of Colloid and Interface Science 371 (2012) 1–14 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis Feature Article (by invitation only) Measurement and analysis of forces in bubble and droplet systems using AFM ⇑ Rico F. Tabor a,b, , Franz Grieser b,c, Raymond R. Dagastine a,b,d, Derek Y.C. Chan b,e,f,1 a Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville 3010, Australia b Particulate Fluids Processing Centre, University of Melbourne, Parkville 3010, Australia c School of Chemistry, University of Melbourne, Parkville 3010, Australia d Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, Victoria 3168, Australia e Department of Mathematics and Statistics, University of Melbourne, Parkville 3010, Australia f Faculty of Life and Social Sciences, Swinburne University of Technology, Hawthorn 3122, Australia article info abstract Article history: The use of atomic force microscopy to measure and understand the interactions between deformable col- Received 29 October 2011 loids – particularly bubbles and drops – has grown to prominence over the last decade. Insight into sur- Accepted 15 December 2011 face and structural forces, hydrodynamic drainage and coalescence events has been obtained, aiding in Available online 27 December 2011 the understanding of emulsions, foams and other soft matter systems. This article provides information on experimental techniques and considerations unique to performing such measurements. The theoret- Keywords: ical modelling frameworks which have proven crucial to quantitative analysis are presented briefly, along Atomic force microscope with a summary of the most significant results from drop and bubble AFM measurements. The advanta- Deformable ges and limitations of such measurements are noted in the context of other experimental force measure- Drops Bubbles ment techniques. Dynamics Ó 2012 Elsevier Inc. All rights reserved. Surface forces 1. Introduction which could be explored, no longer relying on crossed-cylinder ap- proaches between transparent materials. In the quarter-century since Binnig, Quate and Gerber invented Some years later, the concept of extending the measurement of the atomic force microscope (AFM) [1], the instrument has grown colloidal forces using the AFM to include deformable bodies to prominence as a tool for measuring nano-scale topology and emerged. The initial attempts were measurements of equilibrium interaction forces. By monitoring the bending or deflection of a forces between a solid particle on the cantilever and a bubble immo- micro-cantilever, information can be gained on the forces acting bilised on a solid surface [5–8], and later between a particle and oil on the cantilever’s sharp tip [2], with pico-Newton resolution. droplet [9–14]. However, it was soon noted that significantly more When the tip is raster-scanned over a surface, this sensitivity is insight could be gained by picking up an emulsion-scale droplet onto used to provide a 3-dimensional reconstruction of the surface’s the AFM cantilever [15]. This allowed interactions between pairs of form, down to Ångstrom (atomic) resolution. droplets to be examined, where relatively high velocities could be In the early 1990s, it was demonstrated that, by attaching a used to explore hydrodynamic drainage effects. colloidal particle of a few microns diameter to the end of an AFM The information gained in such measurements of inter-droplet cantilever, the force between the particle and a surface could be collisions was underpinned by theoretical analysis [16–19]. A model measured [3]. Using this method, precise information on electrical had been previously developed which accounted for the force seen double-layer and Van der Waals forces was obtained, with specific when a solid particle approached a deformable oil droplet, predict- relevance to colloidal systems [3]. Although the surface forces ing the static force at any approach distance [16,20]. By introducing apparatus (SFA) [4] had previously been able to provide such time as a variable, and accounting for fluid flow by lubrication the- insight for specific systems, the AFM offered several complementary ory, dynamic interactions between droplets could be modelled, advantages in terms of the material combinations and geometries which afforded major advances in understanding [17–19]. This article draws from the significant body of work pertaining to measuring interactions between soft bodies (bubbles and droplets) ⇑ using the AFM. Experimental and theoretical considerations are de- Corresponding author at: Department of Chemical and Biomolecular Engineer- tailed, and key outcomes and advances to date are summarised. This ing, University of Melbourne, Parkville 3010, Australia. Fax: +61 3 8344 4153. is relevant because of (a) velocity and deformation effects that are E-mail addresses: [email protected] (R.F. Tabor), [email protected] (D.Y.C. Chan). important in considering emulsion stability, gel interactions, and 1 Fax: +61 3 8344 4599. soft biological systems, and (b) film drainage studies, where the 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.12.047 2 R.F. Tabor et al. / Journal of Colloid and Interface Science 371 (2012) 1–14 displacement and its time variation are well-controlled as the (time- duced by Prieve [39,40]. In this method, a particle or droplet is held dependent) force of interaction is measured. in position above a surface using an optical trap to limit its lateral (but not vertical) diffusion, and its position measured by an evanes- 2. Scope and intention of the article cent scattering process. Through a Boltzmann analysis, this allows a determination of the potential energy profile of the particle as a This article is intended as an introduction to techniques and function of separation. Although limited to a small range of parti- theories pertaining to performing AFM force measurements specif- cle/drop sizes, forces as low as 10À14 N can be reliably measured. ically on soft, colloidal systems – droplets, bubbles, etc. By force Whilst AFM offers some useful advantages when applied to measurements, we mean the determination of force vs. distance measuring force/separation relationships, there are also limitia- or force vs. time relationships for soft surfaces, as distinct from tions which must be accounted for. The most significant inherent AFM imaging studies, which will not be covered here. It is hoped disadvantage with AFM when specifically applied to soft matter that the experimental methods and approaches detailed herein interactions is that the absolute separation between bodies or will prove useful to other researchers in this area. A summary of interfaces is not explicitly known, and must be inferred. This topic important results and outcomes in the field of force measurements is discussed in more detail in the next section, along with methods on soft colloids provides context and motivation for the work. It is to attempt to overcome the problem. not intended as a significant review, critical or otherwise, of pub- lished work in this field in general. 3. Experimental equipment and methods Measurements of interactions (i.e., forces as a function of sepa- ration) in soft, colloidal systems have attracted much attention over 3.1. Equipment recent decades. However, obtaining such information presents sig- nificant experimental and theoretical challenges, due to the effects 3.1.1. AFM of deformability and the often dynamic nature of colloidal systems. Due to its sensitivity and precision of motion, afforded by piezo- A number of techniques have been succesfully applied to elicit electric elements, the AFM represents an ideal tool for analysing information on surface and other forces from such systems, and interactions between colloidal objects at nanometre separations. each approach presents its own set of advantages and limitations. At such close approach, electrical double-layer, Van der Waals In the 1970s, the surface forces apparatus (SFA), developed by and other, more exotic colloidal forces come into play. Israelachvili, Tabor and Winterton [21–23,4] enabled significant Although all commercial AFMs contain the same basic set of com- advances by measuring interactions between solid surfaces [24], ponents (a cantilever combined with a laser/photodiode setup to successfully measuring electrical double-layer, Van der Waals, measure its deflection, a sample stage and piezo elements to move hydration and structural forces [25]. Horn applied the technique one relative to the other, along with the necessary electronics), the to look at interactions between a solid and a deformable interface, design and implementation of these components differs signifi- in the form of a drop of mercury [26,27] or an air bubble [28] cantly between manufacturers and models. This has naturally led squeezed from a capillary. One advantage of SFA is that by the to some instruments which are highly optimised for atomic-resolu- use of an optical fringe-counting method (‘fringes of equal tion imaging studies, whereas others have strong capabilites for chromatic order’ or FECO), the absolute separation at any time measuring forces. In particular, instruments that have an accurate can be known [4]. Additionally, the spatial profile of the liquid film method for measuring the distance travelled by the cantilever verti- between the approaching interfaces may be obtained, but in this cally are the most
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