Wetting immiscible displacement of one ffuid by another in a porous medium yields self-affine (anisotropic) fractal interfaces between the two ffuids. Water-air interfaces are characterized experimentally by a scale-dependent roughness w(L) ALs, with p 0.73+'0.03, independent of the capillary number Ca, and A cx:Ca a47-. The exponent p is related to the "box" and "divider' dimensions by Dt, 2-P and Dd 1/P, respectively.

The spatiotemporal evolution of field-induced structures in very dilute polarizable colloidal suspensions subject to rotating magnetic fields has been experimentally studied using video microscopy. We found that there is a crossover Mason number (ratio of viscous to magnetic forces) above which the rotation of the field prevents the particle aggregation to form chains. Therefore, at these high Mason numbers, more isotropic clusters and isolated particles appear. The same behavior was also found in recent scattering dichroism experiments developed in more concentrated suspensions, which seems to indicate that the dynamics does not depend on the volume fraction. Scattering dichroism experiments have been used to study the role played by the volume fraction in suspensions with low concentration. As expected, we found that the crossover Mason number does not depend on the volume fraction. Brownian particle dynamics simulations are also reported, showing good agreement with the experiments.

Rubio, Dougherty, and Gollub Reply:In the preceding Comment 1 Horvath, Family, and Vicsek (HFV) report results on experiments suggested to be similar to the ones described in our paper, 2 and compare them with their own reanalysis of our data in Fig. 1 of Ref. 2. They find that the value of the roughness exponent in their experiment is /3=0.88 ±0.08, in agreement with the value obtained by reanalyzing the interfaces in our paper, /3 =0.91 ±0.08. Both of these values differ from our reported result of p =0.73 ± 0.03. They conclude that this discrepancy is particularly important "because it is expected to be relevant from the point of view of universality classes of surface growth phenomena."Because of space limitations, we did not report all of the checks we have made on our roughness calculations, although some have been published elsewhere. 3 As stated in Ref. 2, we also checked our results against direct computations of the box and divider dimensions, and obtained good agreement. Furthermore, we checked all three algorithms on self-affine Weierstrass-Mandelbrot curves for a wide range of roughness exponents. Therefore, we are confident that the results reported in our paper are correct.The discrepancy between our result and that obtained in the reanalysis of our data by HFV is probably due to the process they used to obtain the data. This process included the plotting of the interfaces with a pen of finite width, the printing of the figure, and the redigitization of the data with 740x600 resolution. The first two processes smooth the interfaces on length scales smaller than the actual thickness of the lines in the final printed figure. The final stage involves digitizing the interfaces with higher spatial resolution (in the horizontal direction) than that of the original data.In a system with scaling behavior over many decades, this process should introduce a crossover from rough interfaces at large length scales to smooth ones at smaller scales. In our system, though, where the scaling range is somewhat limited, it is not surprising that the result is simply a higher apparent roughness exponent. Indeed, their "reanalyzed" data do not exhibit as clear a scaling range as the original data. We have checked this hypothesis quantitatively by taking the original data representing the interfaces reported in Fig. 1 of Ref. 2, and processing it in a way that corresponds approximately to that used in Ref. 1. The value obtained for the resulting interfaces was p =0.84 ±0.04, distinctly higher than the correct value we reported. Variations in the details of the interface-finding algorithm might also have some effect. We conclude that a large part of the difference is simply an artifact resulting from the fact that they did not have access to the original data. Thus their reanalysis has little relevance either to our data or to their experimental value.Without additional experimental details, it is difficult to make direct comparisons between our work and the

We report on the orientation dynamics and aggregation processes of magnetorheological fluids subject to rotating magnetic fields using the technique of scattering dichroism. In the presence of stationary fields we find that the mean length of the field-induced aggregates reaches a saturation value due to finite-size effects. When a rotating field is imposed, we see the chains rotate with the magnetic field frequency (synchronous regime) but with a retarded phase angle for all the rotational frequencies applied. However, two different behaviors are found below or above a critical frequency f(c). Within the first regime (low frequency values) the size of the aggregates remains almost constant, while at high frequencies this size becomes shorter due to hydrodynamic drag. Experimental results have been reproduced by a simple model considering a torque balance on the chainlike aggregates.

We present experimental results on the aggregation dynamics of a magnetorheological fluid, namely, an aqueous suspension of micrometer-sized superparamagnetic particles, under the action of a constant uniaxial magnetic field using video microscopy and image analysis. We find a scaling behavior in several variables describing the aggregation kinetics. The data agree well with the Family-Vicsek scaling ansatz for diffusion-limited cluster-cluster aggregation. The kinetic exponents z and z' are obtained from the temporal evolution of the mean cluster size S(t) and the number of clusters N(t), respectively. The crossover exponent Delta is calculated in two ways: first, from the initial slope of the scaling function; second, from the evolution of the nonaggregated particles, n1(t). We report on results of Brownian two-dimensional dynamics simulations and compare the results with the experiments. Finally, we discuss the differences obtained between the kinetic exponents in terms of the variation in the crossover exponent and relate this behavior to the physical interpretation of the crossover exponent.

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops ͑volume of 5-35 l͒ that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed ͑7 cm/s͒ by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

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