- Measuring the structure of thin soft matter films under confinement: a surface-force type apparatus for neutron reflection, based on a flexible membrane approach. WM de Vos, LLE Mears, RM Richardson, T Cosgrove, R Dalgliesh and SW Prescott. Review of Scientific Instruments, 83, 113903, 2012.
Abstract:
A unique surface force type apparatus that allows the investigation of a confined thin film using
neutron reflection is described. The central feature of the setup consists of a solid substrate (silicon)
and a flexible polymer membrane (Melinex R ). We show that inflation of the membrane against the
solid surface provides close and even contact between the interfaces over a large surface area. Both
heavy water and air can be completely squeezed out from between the flexible film and the solid
substrate, leaving them in molecular contact. The strength of confinement is controlled by the pressure
used to inflate the membrane. Dust provides a small problem for this approach as it can get trapped
between membrane and substrate to prevent a small part of the membrane from making good contact
with the substrate. This results in the measured neutron reflectivity containing a small component of
an unwanted reflection, between 10% and 20% at low confining pressures (1 bar) and between 1%
and 5% at high confining pressures (5 bar). However, we show that this extra signal does not prevent
good and clear information on the structure of thin films being extracted from the neutron reflectivity.
The effects of confinement are illustrated with data from a poly(vinyl pyrollidone) gel layer in water,
a polyelectrolyte multilayer in water, and with data from a stack of supported lipid-bilayers swollen
with D2O vapor. The data demonstrates the potential of this apparatus to provide information on the
structure of thin films under confinement for a known confining pressure.
Last edited: Friday September 10, 2010
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