The hydrophobic surface of the leaf leads to the formation of water droplets. Credit:Dreamstime
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In nature many different and fascinating solid-liquid relationships exist – from hairy fishing spiders that skate the surface of their watery hunting ground and the Namib desert beetles with their water attracting protrusions which can pluck water from the air, to waxy lotus leaves who’s water repelling surface helps keep the dirt out. The ability to mimic these kinds of surface phenomena has a benefit to industry and so understanding these solid-liquid surface interactions is an important area of science.
Neutron scattering techniques can reveal the solid–liquid interaction at an atomic scale unravelling many intricate processes such as the wetting of metal by different paints, the ability of oils to lubricate a surface and even how drugs absorb water when they’re ingested. Scientists have recently used ISIS to perform Grazing-Incidence Small-Angle Neutron Scattering (GISANS) experiments for the first time on Sans2d to study the near surface structure of thin film systems. This technique offers unique information that can’t be extracted from other experiments.
When a solid and a liquid come together, the interaction depends on their individual properties - the liquid can either spread itself out wetting the surface, or split into droplets that repel from the surface. Many industrial processes rely on a detailed understanding of this relationship and in particular scientists are eager to find out the kinds of structures which occur at the boundary between a solid and a liquid (the solid-liquid interface).
A group of scientists from Sweden, Germany, France and the UK have been looking at solid-liquid interface of a polymer solution on silicon using GISANS on Sans2d. This is the first time this technique has been used at ISIS.
Dr Max Wolff, Uppsala University, Sweden, led the study, “Scattering techniques can be tuned to become surface sensitive helping us to address some fundamental questions important for the lubrication industry, for example; How does a boundary affect structure in a liquid? How do near interface anomalies affect flow? And how can these effects be used to tailor the properties of liquids and solids? Equally, such techniques can give us an understanding of self-organization at a solid boundary. This can be interesting in many areas like, high density data storage, nano-mechanics components as well as lab-on a chip applications.”
The solid-liquid boundary, however, can be difficult to explore as it sits sandwiched between the solid and the bulk of the liquid. GISANS has the potential to revolutionise this area of research as the geometry used makes the very near surface region more accessible and neutrons, which are highly penetrable, can pass through the solid and reveal what’s happening right at the surface, offering minimal disturbance of the samples under investigation.
“The GISANS technique was pioneered by the thin film community. What makes it different to ‘standard SANS’ is the geometry. Usually with SANS, the beam passes directly through the sample, which is at an upright position, perpendicular to the beam direction. If this geometry is used with thin films, you have a sample which is only a few microns thick, so the scattering signal is really weak and it can be a struggle to get any useful data out of the measurement. GISANS is different as here you reflect the beam across the surface of the sample, which is now lying flat. This allows you to look at a much bigger sample volume due to the increased neutron footprint on the sample and so the scattering signal increases. Additionally, real surface sensitivity can be achieved by exploiting the differing penetration depths of the neutrons with varying wavelength.” Sarah Rogers, Instrument scientist, ISIS.
Typical set up for a GISANS experiment
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The GISANS technique was originally performed at continuous (reactor) sources. Here only one wavelength is used during a measurement and therefore for a certain incident beam angle you concentrate on a specific sample volume. However, time-of-flight (TOF) GISANS at spallation sources like ISIS uses multiple wavelengths in one measurement. With such a setup you probe the surface, near surface as well as bulk properties all in one shot. Equivalent measurements on a reactor are more complicated and involve studying the sample at several incident beam angles taking more time and limiting these studies to systems in equilibrium.
But why use GISANS? Dr Max Wolff explains, “We found that the flow properties of liquids cannot be explained by changes in the local density profile of the liquid close to a solid surface only as probed by standard neutron reflectivity. The local structure and or dynamics needs to be taken into account as well. This is why we are developing grazing incidence scattering techniques to address the correlations parallel to the solid-liquid interface as well. These scattering techniques have the potential to fill in this gap, in particular, as we have shown in our research, if surface sensitivity can be applied..”
The team used GISANS to look at thin polymer films interacting with silicon. In many applications its crucial to understand how a surface affects bulk material; for example if a liquid starts to crystallise under a certain surface it might ruin the bulk material; however you can influence its properties by changing the surface its bound to. In order to study this, the team used two different surfaces of silicon one hydrophilic and one hydrophobic and looked at the interaction at the solid-liquid boundary between the silicon and a self-assembling polymer. They found that the ordering of the surface layer in a liquid critically depends on the properties, such as surface energy or topology, of the solid interface.
GISANS patterns obtained from Sans2d. It can be seen that when changing the nature of the silicon surface from hydrophobic to hydrophilic the structure of the polymer is altered only close to the surface (tops panels) and the bulk structure remains unchanged (bottom panels).
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“In this specific experiment we have shown that depth sensitive GISANS measurements are possible with TOF neutron scattering.” Explains Max, “In particular, we have a direct non-destructive technique to look at solid-liquid boundaries where all the information can be extracted in a single shot experiment. This offers unique information which cannot be extracted from other experiments to answer still open questions about the hydrodynamic boundary condition dating back more than a hundred years.”
The team hope to return to ISIS in the future to use GISANS to study other systems under flow, “In future experiments we hope to link the flow properties in a liquid close to a solid substrate to the local structure and dynamics of the liquid in the near surface region. This should deepen our understanding on the hydrodynamic boundary condition.” Explains Max, “A further large impact might arise if we can show how the local structure and dynamic of water is changed close to solid or soft boundaries. This understanding is highly relevant in biology for understanding the function of drugs as well as the folding of proteins. Regarding this our research will contribute to a better understanding of lubrication as well as that of water dynamics in membranes and open routes for the development of smart liquids with tailorable properties.”
Research date: February 2014
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