Local Ordering of Fluids and Other Nanobeam Application & Waveguides
J. Friso van der Veen1,2, A. Diaz1, C. Bergemann3, O. Bunk1, D.K. Satapathy1, F. Pfeiffer1, C. David1, H. Keymeulen1, Hua Guo4, G.H. Wegdam4
1Paul Scherrer Institut,
Villigen, Switzerland
2ETH-Zürich, Switzerland
3Cavendish Laboratory,
University of Cambridge
4Van der Waals-Zeeman Institut,
University of Amsterdam, The Netherlands
Confinement of a fluid between two opposing surfaces induces ordering of the fluid. This is suggested by surface force measurements [1], showing oscillations in the normal force whenever an integer number of layers are confined. The question arises whether such discrete layering effects can directly be observed by direct structural probes such as x-ray scattering. Another question is whether the confinement induces lateral short-range correlations within the fluid which are different from those in the bulk. In general, knowledge of confinement-induced structural arrangements within fluids is important for understanding lubrication, rheological properties of fluids in narrow tubes and crystallization phenomena.
We report on ordering phenomena within colloids confined within arrays of microcavities that have been lithographically etched in silicon (Fig. 1). The colloid consists of SiO2 particles (112 nm diameter) dissolved in 55% benzyl alcohol + 45% ethanol. Cavity sizes range from 350 to 800 nm. Each array, having a constant period and cavity size, acts as a grating for the x-rays. Scattering from the colloid- filled grating results in Bragg peaks, being the different grating diffraction orders, as well as diffuse intensity. From the Bragg peaks we directly determine the average colloid density profile across the cavity using a one-dimensional phase retrieval algorithm subject to known boundary conditions. The diffuse intensity reveals the short-range spatial variations in the colloid density. The advantage of using a cavity array instead of a single cavity is that the scattered intensity is much higher.
De-ionized colloidal solutions, in which electrostatic interactions are present, reveal average layered density profiles with a period that tends to adjust itself to the cavity width, whereas hard-sphere solutions (with salt added) provide evidence of a ‘fractional-integer’ effect in the number of layers; as the cavity width increases, cycles of layering and disordering occur in succession.
Figure: X-ray scattering setup for
investigations
of confinement-induced ordering phenomena
within colloids.
One would like to scale the confining space down to a width of a few nanometer or less, enabling studies of ordering phenomena down to the molecular scale. It will be very difficult to fabricate nanocavity arrays for this purpose. However, a single planar slit with adjustable gap can be made from two opposing mica surfaces [1]. X-ray scattering experiments in that case would be optimally performed using a beam focused down to less than 10 nm. Clearly, sub-10 nm beams are required for many applications, but they have not yet been realized. We have theoretically investigated the focusing properties of Fresnel zone plate optics [3]. In contrast with x-ray waveguides [2], zone plates possess no fundamental limit to the smallest spot size to which they can focus, provided the zone are tilted such that the x-rays reflect specularly from the zone boundaries.
References:
1. J.N. Israelachvili; Intermolecular and Surface Forces; Academic Press, London (1991)
2. C. Bergemann, H. Keymeulen, and J.F. van der Veen; Focusing X-Ray Beams to Nanometer Dimensions, Phys. Rev. Lett. 91 204801(2003)
3. F. Pfeiffer, C. David, J.F. van der Veen, and C. Bergemann; Nanometer Focusing Properties of Fresnel Zone Plates described by Dynamical Diffraction Theory, Phys. Rev. B, in the press
