Elasticity of Compressed Emulsions
This keyword is currently being edited by Joseph Muth
Title of Original Work: Elasticity of Compressed Emulsions
Journal of Original Work: PRL, Volume 75, Number 10, Sept 4, 1995
Authors: T.G. Mason, J. Bibette, D.A. Weitz
Author of Review: Joseph Muth - AP 225 - Fall 2012 - 11/11/2012
Soft Matter Keywords
Emulsions, Laplace Pressure, Disjoining Pressure
Emulsions consist of a fluid dispersed in a second immiscible fluid where the first fluid is stabilized in the second fluid by a surfactant. Despite the liquid nature of emulsions, the elastic response of such material systems mimics the characteristics of an elastic solid. As a pressure (π) is applied to an emulsion, the volume fraction (Φ) of droplets increases. When the π is sufficiently high so as to force the droplets to touch, Φ approaches one and any subsequent pressure increase results in deformation of the droplets. Deformation leads to stored strain energy and thus elasticity. Deformation of the droplets is governed by the Laplace pressure, σ/r, where σ is the surface tension of the drop and r is the radius of the undeformed drop. Clearly the elastic modulus (E) of the drop will depend on the degree of stored strain energy in the drop interface. Thus if the drops are not touching, then E = 0 because there can be no interfacial deformation. If Φ = 1 then the drops are maximally touching and the emulsion behaves like a dry elastic foam. However, the transition of the emulsions elastic properties between these two limiting cases is not well understood. It is the author's goal to elucidate the elastic property transition of emulsions by measuring the Φ dependence of E using monodisperse emulsions. By creating monodisperse emulsions, differences in Laplace pressure between drops is eliminated - enabling direct comparisons of experimental results with theoretical predictions.
All emulsions were made using silicone oil dispersed in oil. 0.01 M of sodium dodecylsulphate was added as the surfactant. The linear viscoelastic moduli, G' and G, were measured by applying a periodic strain and measuring the stress response.
Figure 1 shows G' and G" as a function of strain for several emulsions of Φ ranging from 0.60 - 0.80. G' increases by four decades over this Φ range. For small strains, G' dominates. At large strains G" dominates. At small strains the emulsion can maintain contact between each drop making the mechanical response more elastic. However, at large strains, the yield limit of the emulsion is reached and flow initiates, thus G" becomes dominant.
Figure 2 shows the frequency (ω)dependence of G' and G" for emulsions with Φ ranging from 0.60 - 0.80. G' remains nearly constant for each emulsion, especially so for higher Φ. However, each G' curve does exhibit some curvature. As such, G' can be calculated at the inflection point - G'p. G" behaves much differently, each G" curve is characterized by a distinct minimum G" value - G"m. The frequency at which G"m occurs increases with increasing Φ. G'p and G"m provide useful reference points for characterizing the elastic and loss moduli, respectively.
To describe the Φ dependence of G'p and G"m for emulsions with different drop sizes, Laplace pressure and disjoining pressure need to be accounted for. Normalizing each result for the Laplace pressure is straight forward because it is governed solely by surface tension and drop size; however disjoining pressure normalization is more complicated. Disjoining pressure is effected by both Φ and the applied pressure because the thin water film between each droplet changes with packing and deformation of the drops. Disjoining pressure is especially significant for film thicknesses (h) of 5-18 nm. As such, an effective Φ (Φeff) must be defined which includes the effects of disjoining pressure such that the emulsion properties alone can be isolated: Φeff = Φ(1+3h/2r). Using this methodology Figure 3 shows G'p and G"m for different emulsions of various Φ and r. It is clear that G'p and G"m both fall on distinct, individual curves. This result indicates that the elastic properties of emulsions only depend on packing geometry of the droplets.
The behavior of the emulsions shown in Figure 3 can be explained by analyzing the extreme limits of the moduli behavior. For Φeff = 1, the emulsion resembles a dry foam. Theoretical predictions for dry foams state that G'=0.6σ/r. The results show in Figure 3 show that for Φeff = 1, G'=0.55σ/r. Thus emulsions can indeed be thought of as dry foams when the emulsion is highly dense.
Further examination of Figure 3 shows a two distinct behavior regions. Above Φeff > 0.63, G' and G" follow one regime, while for Φeff < 0.63, a different type of behavior is followed. This result can be explained in terms of packing mechanisms. Above 0.63, packing constraints govern the droplet arrangement and the energy scaling parameter remains σ/r, but for Φeff < 0.63, packing constraints no longer dictate droplet arrangement. As packing decreases below the critical Φeff value, thermal fluctuations begin to dominate elastic response. More experiments need to be performed to confirm the exact nature of the thermal scaling law, but the transition between these two regimes appears quite acutely in Figure 3.
In most emulsions, the thermal packing regime is of little engineering interest. Thus if we focus on the the elastic response for Φeff > 0.63, an interesting result appears. Figure 4 shows that G'p increases linearly with Φeff when plotted on a logarithmic scale. If we consider a compressed drop as a sphere being squeezed by a cube, then as the cube compresses, flat regimes will appear at the contact points. These facets can be thought of as contact points of a droplet with its eight nearest neighbors. As compression increases, more nearest neighbors come in contact with the droplet - forming new facets.