Capillary Shaping of Solutes

DESCRIPTION AND AIM

This workshop aims to deepen understanding of several striking forms of pattern formation in which capillary flow on a surface shapes a solute deposit and is in turn shaped by that deposit.  A prime example is the robust periodic and hierarchical patterns that appear when a drop of colloidal or nanoparticle dispersion evaporates under suitable conditions.  These patterns are formed by the gradual buildup of a dam or dyke of the solute particles at the contact line.  The contact line then abruptly separates from the solute deposit and begins the process anew.  The interest of this process arises from the robustness and regularity of the observed patterns, suggesting that a) the factors regulating the structure are understandable and controllable and b) further refinement could lead to controlled patterning, coating and response of materials at microscopic scales.  Two puzzles associated with this process are:

•   What conditions determine the rupture event where the fluid separates?  How are these conditions so impervious to random variablilty in the substrate and solute as to produce the observed regularity?

•   What determines the arrest of the rapid motion of the contact line?

Such dynamic capillary shaping of solutes extends far beyond the droplet depinning phenomenon.  Several striking and ill-understood patterns of this nature have recently been identified.  These include:

•   Viscoelastic effects that arise when the solute is polymeric

•   Elastic deformation of the substrate

•   Swelling and buckling of a polymeric substrate that imbibes the solvent

•   The shepherding of solute into regular branched pitchfork structures, seen in both surface evaporation and at fluid interfaces.

•   The rupture of a dense mass of fluid-saturated colloidal particles under negative capillary pressure into robust patterns of fracture cracks.

•   Another form of capillary breakaway in which fluid channels tunnel into the colloidal mass to form labyrinth patterns.

•   The role of surfactants and associated marangoni stresses in controlling the shaping flows.

•   The distinctive buckling of surface layers compressed by underlying capillary flows.

•   The role of differential wetting and flow of mixed solvent.

To add perspective and integration to the discussion of these raw new phenomena, each main talk will be followed by a discussant with complementary expertise.  Participants will also complement the main talks by their questions and ideas in thinking about the puzzles discussed.  All participants will introduce themselves briefly to the group and all are encouraged to present their work in posters and in informal discussion. The workshop aims to focus our thinking on capillary shaping so that our collaboration leads to a new level of understanding and generalization shown in published papers.  It also aims to reveal common interests and begin ongoing relationships within the diverse group of participants.