Microrheology of Aggregated Emulsion Droplet Networks, Studied with AFM-CSLM D. Filip,* V. I. Uricanu, M. H. G. Duits, D. van den Ende, J. Mellema, W. G. M. Agterof, and F. Mugele Physics of Complex Fluids Group, UniVersity of Twente, Faculty of Science and Technology, Associated with the J.M. Burgerscentrum for Fluid Mechanics, and Institute of Mechanics, Processes and Control-Twente (IMPACT), PO Box 217, 7500 AE Enschede, Netherlands ReceiVed August 19, 2005. In Final Form: October 26, 2005 We studied the mechanical behavior of densely packed (up to ∼30% v/v), sedimented layers of (1 µm) water-in-oil W/O emulsion droplets, upon indentation with a (10 µm) large spherical probe. In the presence of attractive forces, the droplets form solid like networks which can resist deformation. Adding a polymer to the oil phase was used to control droplet attraction. The droplet layers were assembled via normal gravity settling. Considering that both the network structure and the droplet interactions play a key role, we used a combination of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) to characterize the mechanical behavior. Here the AFM was used both as indentation tool and as force sensor. Indentation experiments were performed via a protocol consisting of approach, waiting, and retract stages. CSLM was used to observe the network structure at micron resolution in real time. Use of refractive index matched fluorescent droplets allowed the visualization of the entire layer. Upon compression with the probe, a markedly nonhomogeneous deformation occurred, evidenced by the formation of a dense corona (containing practically all of the displaced droplets) in the direct vicinity of the probe, as well as more subtle deformations of force-chains at larger distances. Upon decompression, both the imprint of the indenter and the corona remained, even long after the load was released. The force-distance curves recorded with the AFM correspond well to these observations. For each deformation cycle performed on fresh material, the retract curve was much steeper than the approach curve, thus corroborating the occurrence of irreversible compaction. Contrary to classic linear viscoelastic materials, this hysteresis did not show any dependence on the deformation speed. Our force-indentation approach curves were seen to scale roughly as F ∼ δ 3/2 . The pre-factor was found to increase with the polymer concentration and with the density of the network. These findings suggest that this new AFM-CSLM method could be used for rheological characterization of small volumes of “granular networks” in liquid. Our hypothesis that the mechanical resistance of the networks originates from interdroplet friction forces, which in turn are set by the interdroplet potential forces, is supported by the predictions from a new mechanical model in which the interdroplet bonds are represented by stick-slip elements. 1. Introduction Emulsions form the basis of a wide variety of natural and manufactured materials, including food products, pharmaceu- ticals, biological fluids, agro-chemicals, petro-chemicals, cos- metics, etc. 1 The mechanical properties of emulsions are important in preparing products that can easily flow or that behave like solids (e.g., margarine, mayonnaise). For low volume fractions (well below close packing) and purely repulsive interdroplet interactions, the surface tension ensures that the droplets are spherical in shape, the emulsion as a whole has no elasticity and flows easily under shear. However, in concentrated emulsions, packing constraints force the droplets to slide along each other (dissipating energy via friction) and to deform elastically (storing energy at their interface). This energy is the origin of emulsion elasticity and allows tuning emulsion behavior from fluidlike to elastic solidlike. A review on this subject can be found in ref 2. Another factor that can become more important at high concentrations is (if applicable) the presence of attractive forces between the droplets. Such forces influence both the structures that are formed and their resistance to deformation. In the present paper, we study the mechanical behavior of weakly aggregated emulsions at concentrations which are high enough to cause structural arrest but still significantly lower than the maximum packing fraction in systems with just excluded volume repulsions. The droplets are gelled by adding a biopolymer in the water phase prior to emulsification. For this kind of material, different systems could serve as a reference case: highly concentrated stable emulsions, reversibly aggregated particle suspensions, or granular materials. The macroscopic rheology of concentrated stable emulsions has been extensively documented in the literature. For (close to) monodisperse emulsions, some universal behaviors were reported. Above a critical volume fraction of ≈0.64, associated with the random close packing of monodisperse spheres, a marked change in the rheological behavior occurs. The emulsion then becomes compressed, and starts to behave like a glassy material. 3 In this regime, a low-frequency plateau of the storage modulus G′ was found, with a magnitude that scales quasilinearly with the droplet volume fraction φ. Also the loss modulus G′′ was found to be (almost) frequency-independent at low frequencies. 4 Comparing different emulsions, the magnitude of the G′ plateau was found to show a universal dependence on φ when scaled by the Laplace * To whom correspondence should be addressed. E-mail: d.filip-boar@tnw.utwente.nl. (1) Encyclopedia of emulsion technology; Becher, P., Ed.; Dekker: New York, 1985; Vol. 2. (2) Mason, T. G. Curr. Opin. Colloid Interface Sci. 1999, 4, 231. (3) Mason, T.; Lacasse, M.-D.; Grest, G. S.; Levine, D.; Bibette, J.; Weitz, D. A. Phys. ReV.E 1997, 56, 3150. (4) Buzza, D. M. A.; Lu, C.-Y. D.; Cates, M. E. J. Phys. II France 1995, 5, 37. 560 Langmuir 2006, 22, 560-574 10.1021/la0522653 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/16/2005