Double emulsions are highly structured fluids consisting of emulsion drops that contain smaller droplets inside. Although double emulsions are potentially of commercial value, traditional fabrication by means of two emulsification steps leads to very ill-controlled structuring. Using a microcapillary device, we fabricated double emulsions that contained a single internal droplet in a core-shell geometry. We show that the droplet size can be quantitatively predicted from the flow profiles of the fluids. The double emulsions were used to generate encapsulation structures by manipulating the properties of the fluid that makes up the shell. The high degree of control afforded by this method and the completely separate fluid streams make this a flexible and promising technique.

We demonstrate a novel method for measuring the microrheology of soft viscoelastic media, based on cross correlating the thermal motion of pairs of embedded tracer particles. The method does not depend on the exact nature of the coupling between the tracers and the medium, and yields accurate rheological data for highly inhomogeneous materials. We demonstrate the accuracy of this method with a guar solution, for which other microscopic methods fail due to the polymer's mesoscopic inhomogeneity. Measurements in an F-actin solution suggest conventional microrheology measurements may not reflect the true bulk behavior. PACS numbers: 87.19.Tt, 83.70.Hq, 83.80.Lz Many interesting and important materials such as polymers, gels, and biomaterials are viscoelastic; when responding to an external stress, they both store and dissipate energy. This behavior is quantified by the complex shear modulus, G ‫ء‬ ͑v͒, which provides insight into the material's microscopic dynamics. Typically, G ‫ء‬ ͑v͒ is measured by applying oscillatory strain to a sample and measuring the resulting stress. Recently a new method, called microrheology, has been developed which determines G ‫ء‬ ͑v͒ from the thermal motion of microscopic tracer particles embedded in the material [1,2]. Microrheology offers significant potential advantages: it provides a local probe of G ‫ء‬ ͑v͒ in miniscule sample volumes and can do so at very high frequencies. While microrheology provides an accurate measure of G ‫ء‬ ͑v͒ for simple systems, its validity in common complex systems is far from certain. If the tracers locally modify the structure of the medium, or sample only pores in an inhomogeneous matrix, then bulk rheological properties will not be determined. Such subtle effects currently limit many interesting applications of microrheology.In this Letter, we introduce a new formalism, which we term "two-point microrheology," based on measuring the cross-correlated thermal motion of pairs of tracer particles to determine G ‫ء‬ ͑v͒. This new technique overcomes the limitations of single-particle microrheology. It does not depend on the size or shape of the tracer particle; moreover it is independent of the coupling between the tracer and the medium. We demonstrate the validity of this approach with measurements on a highly inhomogeneous material, a solution of the polysaccharide guar. Two-point microrheology correctly reproduces results obtained with a mechanical rheometer, whereas single-particle microrheology gives erroneous results. We also compare ordinary and two-point microrheology of F-actin [2-4], a semiflexible biopolymer constituent of the cytoskeleton. Different results are obtained with the two techniques, suggesting that earlier interpretations of F-actin microrheology should be reexamined. Conventional microrheology [1,2] uses the equation:wherer 2 ͑s͒ is the Laplace transform of the tracers' mean squared displacement, ͗Dr 2 ͑t͒͘, as a function of Laplace frequency s, and a is their radius. Equation (1) (1) is subject to the same conditions as the...

We present an optical realization of a thermal ratchet. Directed motion of Brownian particles in water is induced by modulating in time a spatially periodic but asymmetric optical potential. The net drift shows a maximum as a function of the modulation period. The experimental results agree with a simple theoretical model based on diffusion.PACS numbers: 05.40.+j Let us consider a Brownian particle diffusing in a one-dimensional periodic well-shaped potential.If the potential height is much larger than the thermal noise, the particle is localized in a minimum. Suppose that this potential is asymmetric and characterized by two length scales Af and Ab (forward and backward) and assume that Ab is larger than Af (time r = 0 in Fig. 1). In an equilibrium situation, not net motion of particles can be induced by a periodic potential, since there is no large scale gradients. However, a time modulation of such a potential, when asymmetric, can induce motion in the following way: Turn the potential off; the particle diffuses freely (time r~r, « in Fig. 1). We call Pf the probability that the particle diffuses forward by more than Af during the time r"«(and similarly Pb for the backward probability).Switching the potential on again after a time~, ff forces the particle to the forward well with a probability Pf and to the backward one with a probability Pq (time r = r,ff in Fig. 1). We define as J = Pf -Pb, the probability current for a particle to advance one step in the periodic potential. Because Ab is larger than Af, Pb is smaller than Pf and the drift is nonzero. As proposed earlier, the time modulation of a periodic asymmetric potential creates directed motion of thermally fluctuating particles [1]. Similar models of engines that extract work from random noise have been recently proposed under the denomination of "thermal ratchets" [2 -6]. These models may have some connection with biological motor proteins [7 -14].How does one experimentally realize such a spatially periodic but asymmetric forcing of Brownian particles?One way is to deposit two metallic films on a glass substrate in a periodic but asymmetric fashion, so that applying an ac electric field through these electrodes creates the desired potential for colloidal particles in an aqueous solution.Recent experiments using such a setup confirmed the induced drift [15,16]. However, hydrodynamic interactions and the complicated electrical response of charges in water limited these experiments to only qualitative agreement with theory. In this Letter, to avoid hydrodynamic interactions we study only one particle (a 1.5 p, m diameter polystyrene 7='Toff Pb (1-Pb-Pf) Pf FIG. 1. The asymmetric potential is drawn as the thickline. The forward and backward length scales defining the asymmetry are Af and Ab. The particle probability densities are drawn as thin lines. At time 7-= 0, the particle is localized and the probability density is sharply peaked. For times 7.~~, «, the potential is off and the particle diffuses freely. At time T 7 ff the potential is back on and t...

We develop a multiple particle tracking technique for making precise, localized measurements of the mechanical microenvironments of inhomogeneous materials. Using video microscopy, we simultaneously measure the Brownian dynamics of roughly one hundred fluorescent tracer particles embedded in a complex medium and interpret their motions in terms of local viscoelastic response. To help overcome the inherent statistical limitations due to the finite imaging volume and limited imaging times, we develop statistical techniques and analyze the distribution of particle displacements in order to make meaningful comparisons of individual particles and thus characterize the diversity and properties of the microenvironments. The ability to perform many local measurements simultaneously allows more precise measurements even in systems that evolve in time. We show several examples of inhomogeneous materials to demonstrate the flexibility of the technique and learn new details of the mechanics of the microenvironments that small particles explore. This technique extends other microrheological methods to allow simultaneous measurements of large numbers of probe particles, enabling heterogeneous samples to be studied more effectively.PACS number͑s͒: 83.85. Ei, 83.10.Pp, 82.35.Pq, 62.25.ϩg