The atomic force microscope (AFM) can be used to image the surface of both conductors and nonconductors even if they are covered with water or aqueous solutions. An AFM was used that combines microfabricated cantilevers with a previously described optical lever system to monitor deflection. Images of mica demonstrate that atomic resolution is possible on rigid materials, thus opening the possibility of atomic-scale corrosion experiments on nonconductors. Images of polyalanine, an amino acid polymer, show the potential of the AFM for revealing the structure of molecules important in biology and medicine. Finally, a series of ten images of the polymerization of fibrin, the basic component of blood clots, illustrate the potential of the AFM for revealing subtle details of biological processes as they occur in real time.

Measurement of the fractal dimension, D, of colloidal aggregates of small silica particles is reported. We observe power-law decay of the structure factor [S(k) -k o] by both light and x-ray scattering showing that the aggregates are fractal. D is found to be 2.12+0.05, which is intermediate between recent numerical results for the kinetic models of diffusionlimited aggregation (D = 2.5) and cluster aggregation (D = 1.75), but is rather close to the value for lattice animals (D = 2.0), which are equilibrium structures. PACS numbers: 64.60.Cn, 05.40.+j, 61.10.Fr Understanding aggregation has been a primary goal in the field of colloidal physics for many years. 'In addition to its importance in commercial processes, aggregation is a prototypical example of a complicated random process which may display such features as self-similarity, scaling, and universality. 2 These features have been revealed by computer simulations.It has been shown recently, for example, that the two most popular models, diffusion-limited aggregation3 4 (DLA) and cluster-cluster aggregation 6 (CA), produce ramified structures that are self-similar in that the two-point density-density correlation function p2(r) is of a power-law form, p, (r) -r ", for values of r intermediate between the lattice constant or monomer size a and the cluster size R. Structures described by Eq. (1) are self-similar and are known as fractals; their essential geometric properties are independent of length scale. In ddimensional space, they are characterized by a fractal or Hausdorff-Besicovitch dimension D related to A by D = d -A. An immediate consequence of Eq.(1) is that the radius of gyration of a cluster RG is related to the number of particles it contains N, by N, -RD (2) A uniform object has D = d, while more open structures in which the density decreases with distance from the center have D & d. Numerical simulations have shown D to be -Sd/6 for DLA in d dimensions for both lattice (2~d~6) and nonlattice (d=2, 3) diffusion, independent of sticking coefficient s (0. 1» s~1). Cluster aggregation in which many particles diffuse and stick together to form clusters which also diffuse and stick yields self-similar aggregates having D =1.45 and 1.75 in two and three dimensions, respectively.

Spatial scale invariance represents a remarkable feature of natural phenomena. A ubiquitous example is represented by miscible liquid phases undergoing diffusion. Theory and simulations predict that in the absence of gravity diffusion is characterized by long-ranged algebraic correlations. Experimental evidence of scale invariance generated by diffusion has been limited, because on Earth the development of long-range correlations is suppressed by gravity. Here we report experimental results obtained in microgravity during the flight of the FOTON M3 satellite. We find that during a diffusion process a dilute polymer solution exhibits scale-invariant concentration fluctuations with sizes ranging up to millimetres, and relaxation times as large as 1,000 s. The scale invariance is limited only by the finite size of the sample, in agreement with recent theoretical predictions. The presence of such fluctuations could possibly impact the growth of materials in microgravity.

We report experiments on convection patterns in a cylindrical cell with a large aspect ratio. The fluid had a Prandtl number σ ≈ 1. We observed a chaotic pattern consisting of many rotating spirals and other defects in the parameter range where theory predicts that steady straight rolls should be stable. The correlation length of the pattern decreased rapidly with increasing control parameter so that the size of a correlated area became much smaller than the area of the cell. This suggests that the chaotic behavior is intrinsic to large aspect ratio geometries.

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