Spreading and Wetting

Placing a droplet of a liquid on a solid substrate can have quite different consequences:

A.  No wetting at all, which can result in an easy rolling-off of the droplet at small tilt angles (see Lotus-Effect)
B.  Partial wetting and formation of a somewhat flattened droplet
C.  Very fast and complete spreading (called superspreading).

The situations A and B can be desribed in terms of contact angle (Young’s equation); the spreading process C, however, is more a matter of spreading kinetics.

Considering Young’s equation, the most important parameter controlling wetting is the surface tension γ of the liquid. Whereas oil phases or aqueous solutions of organic surfactants exhibit surface tensions of around 30 mN/m, silicone surfactants can provide surface tensions as low as 22 mN/m to both oil- or water-based systems.

Therefore, wetting of both oil- or solvent-based systems (e.g. see Agro web page) and of aqueous systems (e.g. adhesive dispersions) can be greatly improved by using silicone surfactants (see Functional Materials, Tegopren, OMS)

Superspreading:
An attribute of a select number of nonionic trisiloxane surfactants which permits a small droplet of an aqueous solution, when applied to a hydrophobic surface, to immediately spread into a thin film covering an area up to 200 times greater than the original droplet. For example, if a 50 µl droplet of a 0.1% w/w solution of the proper trisiloxane surfactant is placed onto a polyolefin film, in less than one minute the droplet will form a thin water film of 80 mm in diameter. In other words, the area covered by this droplet will expand from 30 mm² to 5000 mm².

This superspreading is pretty amazing to observe; it finally leads to a “pancake” of water only 10 µm thick (using the experimental parameters given above).

The low surface tension typical of trisiloxane surfactants is a necessary condition to obtain superspreading; however, the low surface tension is not the cause of the fast spreading process! We do have trisiloxane surfactants in our product range showing the same low surface tension which do not superspread! It has been found that the superspreading process is related to the phase behaviour of the surfactants in water: Trisiloxane surfactants forming spherical micelles in water (like many “ordinary” surfactants) do not superspread, whereas trisiloxanes forming bilayer aggregates such as vesicles or lamellar phases do superspread.