In the context of measurements using hydrophones, wetting critically affects the coupling between the hydrophone encapsulation material (in particular, that surrounding the active element) and the measurement medium (assumed in this case to be water). Wetting is the readiness of a liquid to bond to the surface of a solid. The process is governed by the relative surface energy of the solid and the surface tension of the liquid.
Device preparation and wetting
By ensuring that the hydrophone boot material is cleaned prior to use, surface contamination will be minimised and the likelihood of poor wetting will be reduced. A mild detergent will usually be sufficient for cleaning general dirt and grease. If the device is new, there can sometimes be a residue left by the mould release agent used in manufacture, and this can often cause wetting problems. Dependent upon the boot material, gently wiping the surface with an alcohol soaked cloth will remove the mould release agent residue. Great care must be taken not to damage the hydrophone through excessive use of alcohol (or any other solvent). It is recommended that a small area away from the active element be tested beforehand.
The mechanism of wetting
Wetting is the readiness of a liquid to bond to the surface of a solid. The process is governed by the relative surface energy of the solid and the surface tension of the liquid.
Surface energy is the energy possessed by the atoms at the surface of the solid, whereas surface tension is the result of the attraction between molecules at the liquid surface. For a liquid to wet a solid, the surface energy of that solid must be able to overcome the surface tension of the liquid, thus breaking the surface tension and forming a permanent film that will bond to the surface.
Any change in conditions can upset the balance between a suitable surface energy and surface tension. For example, simply changing the temperature of the liquid will have an effect on the ratio between surface energy and surface tension. A more marked effect would be seen if the surface of the hydrophone became contaminated with grease or oil, even from simply touching the device. The grease dramatically lowers the surface energy of the hydrophone (the solid), potentially to the point where the surface energy is insufficient to break the surface tension of the liquid.
Application of the surfactant
A surfactant can also be used to aid the wetting process. A surfactant (surface active agent) when added to a liquid will reduce the surface tension of the liquid, thus improving its wetting abilities.
For practical purposes, simple household washing up liquid makes for a good surfactant to use on hydrophones. It is readily available, cheap, non-toxic and reasonably mild in strength.
The most suitable way to apply it is to make a mix of water and washing up liquid of about the same strength as when washing household crockery and apply it to the hydrophone with a soft to medium brush. Once applied the device should be placed into the measurement water straight away. This will prevent a thin film of air being trapped between the device and the water. It can be used in its neat form to clean the device, but should then be washed off.
Function of a surfactant
A surfactant consists of a molecule with two parts, a water-soluble portion and an oil-soluble portion. The oil-soluble portion (the hydrophobe) is a hydrocarbon chain usually consisting of between 8 and 18 carbon atoms. The water-soluble portion is called the hydrophile. There are four distinct varieties of surfactant, these being anionic, cationic, nonionic and amphoteric, the difference being in the charge of the hydrophilic head: anionic has a negative charge; cationic a positive charge; nonionic has no charge; and amphoteric can exhibit both charges.
When the surfactant is added to water, it will diffuse throughout its volume. The shorter the hydrocarbon chain is, the more readily the molecules can diffuse to the surface. Once at the surface a bond can be made between the water surface and the solid surface, with the surfactant acting as the interface, effectively reducing the surface tension of the liquid.
Although a short hydrocarbon chain allows for rapid diffusion to the surface, the bond it forms is quite weak and easily washed away. A very long chain, although able to make a strong bond, can take too long to diffuse for many applications.
It is not recommended that the surfactant be added directly to the water without first seeking expert professional advice and specialist products. Although detergents often contain a surfactant, they can also contain other ingredients such as foaming agents. When applied to the device in the manner described above, these other ingredients present few problems. However, applied to a much larger volume of water it is likely cause significant problems due to the action of the foaming agents, especially if the water is agitated (for example by the pumps in re-circulation and filtration systems).
Another problem associated with wetting that can occur is gas coming out of solution and clinging to the hydrophone surface whilst submerged. This can happen if there is a large enough temperature difference between the water and the device under test. If the device is warmer than the medium when submerged, the water in close proximity to the hydrophone will be heated slightly, the rise in temperature causing dissolved gas to come out of solution and form small bubbles. These bubbles can then stick to the hydrophone and cause measurement problems. This problem can be avoided by pre-soaking the device in the test tank (or in water which is at the same temperature as that in the tank). The length of time required for pre-soaking will depend on the device under test and the temperature difference, but at least half an hour soaking before the start of measurements is advisable (though some devices may require much longer). It is recommended that the soaking time be reported along with the results of a calibration.
Effect on measurements
The adjacent figure illustrates the impact wetting can have on calibrations. In the figure, results are shown of two calibrations of the same device. The blue curve shows the results with the hydrophone fully wetted, and the red curve shows the results obtained when the surface of the hydrophone was severely contaminated by surface grease. In general, wetting effects may vary with frequency and often the effects are time dependent, leading to measurements that are unstable with time.
Also shown is a polar plot of the directional response of a hydrophone measured before and after temperature stabilisation. The hydrophone was initially placed into the tank without being wetted with detergent and measurements were made without allowing the temperature of the device to stabilise. Bubbles were clearly visible on the surface of the hydrophone boot when viewed through the viewing window of the tank, and the polar response measurement resulted in the blue curve shown. The bubbles were brushed away and the hydrophone was allowed to soak for some time before the measurements were repeated, resulting in the green curve shown. The red curve is the result of measurements made on a wetted hydrophone which had been soaked in the tank to allow the temperature to stabilise fully, and these results represent the expected directional response of the hydrophone. It is clear that bubbles clinging to the hydrophone can significantly affect the measured response.
For further information, contact Justin Ablitt