Spreading dynamics of polydimethylsiloxane drops: Crossover from Laplace to van der Waals spreading

The spreading dynamics of small polydimethylsiloxane (PDMS) drops was studi ed on substrates with varying surface energies. For experimental parameters near the wetting transition, we observed small PDMS drops of different dro p volumes as a function of time using interference video microscopy. While for large drops the contact angle theta decreases with the well-established power-law relation theta similar to t(-0.3) (Tanner's law), the effect of dispersive van der Waals (VW) interactions must be taken into account when interpreting the evolution of small drops. Two signatures of the VW forces are observed. For a positive Hamaker constant, the disjoining pressure acts as an additional driving force, leading to an acceleration of droplet spre ading as soon as the drop height becomes comparable to the range of the VW interactions. In addition, a precursor film forms ahead of the contact line , leading to an apparent volume loss, particularly noticeable for very smal l drops. Contact line pinning may be a problem and we describe its effect o n our experimental results. We present a theory that discusses the interpla y of surface tension and VW forces in the case of a spreading drop. This mo del predicts a new spreading regime for very thin drops, in agreement with our experimental results. (C) 2001 Academic Press.