When a drop of water is placed on a surface it can either spread over the surface in which case it wets or can form a stable drop and is then unable to wet.
Reduction in surface tension of water by a surfactant can make a non-wetting solution into a wetting solution on particular substrates. The ability to wet or not depends upon the surface tension of the solution and the critical solid tension (CST) of the solid. The CST is the surface tension of a liquid which will form a contact angle of close to zero measured across the film on that solid, i.e. the liquid spreads over the solid. Glass, with a CST of more than 70 dynes/cm can be wet easier than polypropylene, with a CST of 28. In all cases to wet a substrate quickly and effectively, a surfactant will be required to help reduce the surface tension and overcome the surface energy of the substrate.
Below depicts the droplet on a substrate with increased wetting performance.
Static contact angle measurement
One of the most popular ways of measuring contact angles is the use of measuring the angle of a sessile drop resting on a solid surface, using a goniometer equipped with a microscope and video camera.
Basically, the drop of the prepared test liquid is released through a fine needle and allowed to drop under gravity onto the substrate whilst filming the action. From the captured image as seen in the first image above the contact angle can be measured either manually or by utilizing the software.
Sessile Drop A Typical Goniometer
Experimental work on various substrates
Below will highlight of the programme of work that we have carried out on various substrates and substances.
Several test liquids were prepared as a 0.5% solution in deionised water and some test substrates consisting of different plastics and metals were utilised.
An FTȦ 200 goniometer equipped with a digital microscope and video capture was used to analyse the sessile drop images.
A detailed list of substrates tested and their recommendations:
The following photo stills show the test liquid sessile drop after 5 seconds of wetting on a variety of substrates: