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Anomalous viscous loss in emulsions

An emulsion is a dispersion of liquid droplets suspended in a second, immiscible liquid that contains a surfactant to stabilize the interfaces. Because they are deformable, droplets can be packed to very high volume fractions while still retaining integrity. These concentrated emulsions are remarkable because they are highly elastic, even though they are made up entirely of liquids [1,2]. They also pos­ sess viscous properties; this makes them viscoelastic. Such materials are often characterized by their complex dynamic shear modulus, G*((o) = ΰ'(ω) + iG!'(co), where G'{cû) is the elastic or storage modulus and G"(æ) is the vis­ cous or loss modulus. These moduli reflect the in-phase and out-of-phase responses to a small oscillatory shear at frequency ω. For concentrated emulsions, G'(a)) is large compared to Gn(a>) at low frequencies, reflecting their elastic nature. However, G'(co) is anomalously low com­ pared to any reasonable expectation based on the droplet deformations [2]. By contrast, G"{a>) is anomalously large compared to any reasonable expectation based on the fluid viscosities [3]. While most work to date has focused on the storage modulus [ 1,2,4-7], the behavior of the loss modu­ lus is equally important and interesting, and many dissipa­ tive mechanisms have recently been considered [3].