This meeting sought to improve the understanding of a novel class of kinetic material-shaping phenomena. The material is a colloidal or macromolecular solute in a liquid solvent. The phenomena arise from capillary flow, dictated by the liquid's surface tension. Such flow can be induced for example by evaporation or by gravity. The workshop considered principally surface flows. The novelty arises because of the reciprocal interaction between the capillary forces on the one hand and the adhesive and cohesive forces of the solute on the other. Thus solute particles may be swept along the substrate by the capillary flow. But once anchored on the substrate, eg at a contact line, the solute particles constrain the shape of the interface and thence the flow field.
A simple example is the drying of a drop of dirty water on a surface. The colloidal dirt becomes strongly concentrated at the contact line, forming a characteristic and generic concentration profile. Altering parameters of the solute or substrate leads to a rich variety of different patterns: radial pitchfork deposits, azimuthal concentric rings, and multiscale patterns resembling a Sierpinski gasket. The workshop dealt with several deposition geometries and several substrate effects as mentioned below.
The meeting sought to identify common structural elements and common pattern-forming mechanisms and limiting principles. In particular, it sought to identify features caused by quasi-static forces and those caused by kinetic forces where the rate of flow and deposition are crucial.
Meeting size and format
The one-week meeting attained its target size of 50 participants. By active selection and recruitment, we were able to attract the people needed to study the diverse phenomena of the workshop. Discussion centered on only three overview talks per day. Each speaker was followed by a pre-arranged discussant: speaker, discussant's remarks and audience discussion were scheduled for 90 minutes of time. All participants were invited to contribute a poster. The meeting began with a brief self-introduction by every participant. Virtually all the participants were active as speakers, discussants, poster presenters or session chairs. The meeting was leavened with an opening wine-and-cheese party and a dinner excursion. A final session aimed to state what learnings had emerged from the meeting.
The workshop staff actively developed an online presence for each participant. The workshop web site had a place for each participant's picture with links to their introductory slides, their presentation slides and their own web sites.
This format seemed to be successful. The tone and atmosphere of the meeting was very interactive with many lively discussions. Many participants emphasized afterwards how much the meeting had been valuable for them. Though no one specifically praised the web site arrangements, we think the accessibility of participants' work via this site helped to build the momentum of the discussion.
Major developments and findings
Mechanism for stick-slip behavior
Many of the patterns featured periodic oscillation between a stationary (stuck) contact line and a moving (slipping) one. Early in the conference, stick-slip was attributed to a simple static force balance.
But at the end a kinetic mechanism dominated by viscous forces seemed to emerge as the more generic mechanism. The importance of viscous forces for influencing the solidity of the deposits and thus their resistance to later capillary forces was noted.
Buckling and chevrons
Two groups reported a puzzling chevron pattern in the drying of a concentrated suspension. The chevrons are regular parallel stripes oriented at 45 degrees to the flow direction and to the deposition front. Since the two groups used different formulations and geometries, both were very interested in the leverage to be gained by comparing the experiments. Both groups agreed to pursue their work in order to decide whether the chevrons were a form of buckling, a density wave or a shock-front phenomenon.
Cracks and labyrinths
At least two groups studied deposition of dense suspension between parallel plates. They found crack patterns resulting from capillary failure of the fluid air interface. One puzzle is to explain the separation of the cracks that form. A possible mechanism was proposed based on the difference between the density of the uniform deposit versus the close-packed density.
Appraisal, impact and recommendations
Any meeting of this sort helps to advance its field by keeping participants abreast of one-another's work and challenging the speakers' assertions. Some meetings go beyond this level to develop a sense of coherence and direction in the field. The organizers had the impression the this workshop achieved some of this coherence. We believe that the impact of this coherence will emerge in the next year or two in publications addressing the puzzles noted above. The workshop aimed to identify the diverse deposition effects as a common body of phenomena amenable to common ways of understanding. We believe the workshop promoted this view, though some partipants argued forcefully that each phenomenon needed to be treated on its own terms.
We found the online aspect of the workshop useful and worth repeating in future workshops. It gives ready access of each participants work and viewpoint for the other participants to see and engage with each other.
We found the speaker/discussant format borrowed from previous Lorentz-center meetings useful and worth emulating.
We liked the use of an self-introduction session on the first day of the workshop. The participants used their introduction time to good advantage, and it gave everyone a sense of the others. These introductions were particularly valuable for this meeting, where most participants didn't initially know one another.
Some conclusions seemed to emerge with clarity during the meeting. We summarized these above. However, it would have been beneficial to articulate these findings in convincing detail, citing specific phenomena and quantitative mechanisms. We regret that we didn't take the opportunity to write a review article detailing these findings. However, we still have our notes and memories, and there is still a prospect that such an article can be written.