Surface force measurement at high hydrostatic pressure
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Abstract
Surface forces are well understood at standard ambient conditions, but there is very little understanding about surface forces at high hydrostatic pressure. The main cause for the lack of understanding is the absence of a proper tool, most likely due to technical difficulties associated with high pressure. However, many systems that are dominated by surface forces occur in high pressure environments. Such environments are e.g. the deep sea or bore holes for oil recovery. Thus, it is desirable to open up the research field of surface forces at high pressure to experimental investigation.
In this work I developed a scientific instrument for surface force measurement at high hydrostatic pressure. The instrument is a combination of a long-working-distance optical trap, an interferometer and an optical high-pressure cell. Surface forces were measured between a glass wall and a colloidal glass bead in aqueous solutions. The bead was pushed against the wall by the optical trap. The distance between bead and wall was determined by evaluation of the reflection interference between bead and wall. The entire system was placed inside the optical high-pressure cell that allowed for the realization of up to 1 kbar of pressure, which corresponds to the highest pressure encountered in the deep sea.
The instrument was applied to the investigation of the pressure dependence of the electrostatic double-layer force. Force-distance curves were recorded with sub-nanometer distance resolution and a maximum force of the order of 0.1 nN. It could be shown that the effect of pressure on characteristics of the electrostatic double-layer force is minor. The pressure effect could be traced back to the pressure dependence of the dielectric constant of water. Other pressure effects, e.g. due to a change in the zeta-potentials of the
electric double-layers were excluded. The observation of only minor and explainable changes in the recorded force curves with pressure demonstrates that the instrument works reliably at high pressure.
Furthermore, the thermal motion of the bead close to the wall was investigated. It was found that the thermal motion of a colloidal bead can be recorded with nanometer resolution at high pressure with the developed instrument. This unique capability gives the possibility for widening the range of interactions that can be studied at high pressure and improving the force resolution to the order of femtonewton.
The results demonstrate that the developed instrument is a suitable tool for the investigation of surface forces at high hydrostatic pressure. The instrument is applicable to a multitude of interactions and will certainly deepen our understandings of colloidal systems in high pressure environments.