Controlling drop morphology : theory, experiments and applications in printing, self-cleaning coatings and micro-fluidic systems

Abstract

The accurate control of drop morphology as a drop is placed on a solid surface is an important prerequisite in many applications, such as (inkjet) printing of functional materials, micro-fluidic devices and smart coatings. Carefully patterning the surface on micrometer length scales and combining this with controlled drop placement is shown to allow the creation a variety of drop morphologies in both simple and complex (liquid crystalline) fluids, which is an important parameter in the above-described fields of interest. Starting from the governing thermodynamic equations determining the morphology of drops, the dominant energetic terms are identified for the different length scales of both the drop sizes and micro-structures in the substrate surface. For non-liquid crystalline fluids, these are gravitational potential energy, surface energy and contact line energy. To study the influence on the wetting behaviour on various patterned surfaces, the object of analysis was chosen to be a drop (or droplet) smaller than 1 mm in linear dimension. A combination of experiments, numerical modelling and theoretical analysis is used to explain the oftensurprising drop shape morphologies and their dependence on the deposition method. On surfaces patterned with parallel grooves (i.e. a corrugated surface), drops were found to elongate parallel to the grooves if the drops were deposited using a non-contact method, such as via inkjet printing or careful placement with a needle. However, if the drops were positioned in an overspread position (such as when pressed onto a surface with a contact printing technique such as micro-transfer printing), the drops elongate perpendicular to the corrugations. The key difference is that hysteresis due to contact line pinning is almost completely absent parallel to the corrugations and is present and significant perpendicular to them. Microtransfer printing with nematic thermotropic liquid crystal monomers leads to similar perpendicular elongations under similar experimental conditions, even when the energetic contributions due to the alignment of the liquid crystal director favour elongation parallel to the corrugations in the direction of alignment. Drops of water are shown to be able to exhibit a transition between two important wetting states by employing corrugated surfaces, combined with electrowetting and a high intrinsic contact angle of the surface. The transition from the collapsed (Wenzel) state to the suspended (also known as Cassie-Baxter) state was observed experimentally for the first time without having to heat the drop above the boiling point in order to lift it out of the corrugations. The mechanism of this lifting transition is also investigated in detail with numerical simulations. The analysis shows that only under carefully chosen conditions, which require the elimination of contact line pinning, it is possible to have such a transition spontaneously without other forces such as vibration are employed. The number of achievable morphologies of drops is extended to non-intuitive shapes such as octagons, hexagons, squares and quasi-triangular by employing surfaces patterned with micrometer sized posts. The modulation of the lattice according to which these posts are placed, as well as the shape of the posts itself, creates various drop shapes as the interface de-pins from the posts differently in different directions, also dependent on whether the drop is spreading or retracting. Experimental inkjet printing is combined with microscopy and numerical simulations to elucidate the local pinning of the interface. An important application of smart coatings is self-cleaning materials in for instance windshields, textiles or ship hulls. Liquid repellent surfaces are a particular example with great industrial relevance. An analysis of the stability of the suspended drop states is presented by employing a recently created experimental surface containing raspberry-shaped silica particles covered with lyophobic polymers. By carefully studying the complex wetting states possible and the transitions between them, design rules for stable liquid repellent surfaces are derived. The method of analysis is generalised so that in the future further surfaces can be analyzed in similar fashion. Finally, a number of new potential applications are discussed in a technology review, where also a view to future developments in the field is briefly discussed.