4 bility or diffusivity of a dilute particle assembly confined in a closed ring-like microfluidic channel. We thank E. Rabani for help in the simulations and T. Beatus, S. Bhattacharya, J. Blawzdziewicz, S. Rice, and T. Tlusty for helpful discussions. This research has been supported by the Israel Science Foundation (ISF) under Grant No. 588/06. DF acknowledges partial sup- port from the Israel Council for Higher Education. 2 4 6 8 10 d / h 0 0.2 0.4 0.6 0.8 D - col  /  D 0 1 1.5 d / h 0 0.01 0.02 n D c ( nd )/ D 0 FIG. 3. Collective diffusion coefficient of an array of particles as a function of interparticle separation. Lattice-Boltzmann simulation results (solid) are presented alongside the analyt- ical result [Eq. (5); dashed]. The inset shows the collective coefficient as expected from the hydrodynamic-screening de- scription. The particle diameter is σ = h/ 1 . 5. of a channel of length d and imposing periodic boundary conditions in the x direction. The collective mobility is then obtained from the net displacement (time-integrated velocity) of the particle in response to an initial impulse. As shown in Fig. 3, the simulation results are in good agreement with Eq. (5), exhibiting a slow 1 /d decay. (No fitting parameters are used in Fig. 3 in addition to the already extracted values of α ≃ 28 and γ ≃ 2 . 61.) In summary, the common view, according to which particles move through liquid-filled channels and pores in an essentially uncorrelated way, is quite misleading. Before steady state is reached, the motions of confined particles are in fact correlated over large distances and long times. The physical mechanism behind the correla- tions is fundamentally different from that in the uncon- fined case, as it originates from the confinement-affected longitudinal (rather than transverse) liquid response [18]. Consequently, the long-ranged correlations are negative (rather than positive) and do not give rise to correspond- ingly long-ranged steady correlations, so that at steady state hydrodynamic screening is recovered. These long- ranged temporal correlations may have important impli- cations, for example, for suspended particles in porous media or the dynamics of polymer translocation through narrow pores. Despite the small factor D − 1 / 2 s ∼ c − 1 s appearing in Eq. (4), they should be observable when probing sufficiently short times. For instance, over a mil- lisecond (kHz frequency) Eq. (4) predicts for particles in a micron-wide channel filled with water a velocity cross- correlation of order − 10 − 2 ( µ m/s) $^{2}$, independent of par- ticle separation . In addition, the predicted strong effect that arises from the uniform longitudinal mode [Eq. (5)] can be readily checked by measuring the collective mo- [1] W. B. Russel, D. A. Saville, and W. R. 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