NSF’s ChemMatCARS held a workshop on November 21-22, 2019, at the Advanced Photon Source, Argonne National Laboratory, to plan new initiatives in X-ray scattering from liquid interfaces at NSF’s ChemMatCARS’ planned second beamline. The purpose of the workshop was to evaluate options for expanding the liquid interface program into areas including liquid-solid interface scattering for the study of biomembrane and cell-surface processes (among other areas) and coherent and transmission interface scattering for the study of interfacial heterogeneities. Also under consideration was an expansion in capabilities for our ongoing studies of liquid-vapor and liquid-liquid interfaces. Once the attendees were chosen, the subject matter and topics were driven by their scientific interests.

The workshop was organized along three topical areas: 1) biomembranes, 2) cell-surface interactions, and 3) heterogeneities, drops, flows, and other topics. A series of talks filled two thirds of the first day. After an introduction to NSF’s ChemMatCARS and an introduction to new initiatives in liquid interface studies, invited talks explored the three topical scientific areas and two technical areas envisioned for new initiatives. Please see the agenda for details.

The remainder of the workshop consisted of the activities of three discussion groups, each focused on one of the three topical areas. The initial assignments to these three groups are shown on the Attendee List; most people stayed exclusively with their group, though the opportunity to move between groups was available to them.

Initially the groups provided the opportunity for all attendees who had not presented invited talks to present a 5-minute overview of their research interests relevant to the workshop. Afterwards, the group leaders led the group through a directed discussion guided by the Group Discussion Guide Form. These proved to be lively discussions that contained a range of scientific ideas and the possibilities for new experiments at NSF’s ChemMatCARS. Subsequently, preliminary reports were written by each of the three groups summarizing the discussion. Finally, the leader of each group presented an overview of the group discussions to an assembly of all workshop participants.

Very brief summaries of the three group discussions are presented below.

Biomembranes. The biomembranes group was led by Ka Yee Lee of the University of Chicago and consisted mostly of experts currently involved in the characterization of biomembranes with X-ray and neutron scattering techniques. This group provided detailed and numerous suggestions for experimental topics, hardware and X-ray technical capabilities, required facilities for sample preparation, suggestions for data analysis capabilities, ancillary characterization techniques, staffing, and accessibility for non-expert users. Different sample configurations were discussed, including bilayers at the solid-liquid interface, multilayer stacks of lipids, droplet interface bilayers, and innovative patch clamp studies. Many detailed suggestions and interested attendees provide the basis for the development of this area at NSF’s ChemMatCARS.

Cell-Surface Interactions. This group was led by Luka Pocivavsek of the University of Chicago and consisted mostly of experts in cell-surface interactions, including a contingent of surgeons interested in basic research questions. Many members of this group were not experienced with X-ray scattering, but a few experts took part and helped to guide the discussion. The goals of this group included determining if X-ray scattering is a feasible method to investigate cell-surface interactions and to design proof-of-principle experiments to test ideas before the advent of the new facilities on the second beamline at NSF’s ChemMatCARS. Two promising areas were identified: 1) measurements of cell/material, bio-scaffold/liquid surfaces and interfaces under dynamic loads under physiologic conditions, and 2) flow and mass transfer in near-cell layers. This group provided suggestions for experimental topics, hardware and X-ray technical capabilities, required facilities for experimental setups and systems, sample preparation facilities, ancillary characterization techniques, and desired X-ray techniques. Much discussion took place on designing proof-of-principle experiments, which several of the members of this group plan to pursue.

Heterogeneities, Drops, Flows, and other topics. This group was led by Mark Schlossman of the University of Illinois at Chicago. Participants in this group were interested in a wide range of subject areas, including interfaces of boiling liquids, thin-film stratification, 2d materials growth, nano-lattice structures, protein-surface interactions, surfactant herding, and novel 2d macromolecules. They were interested in the capabilities of small beams to probe interface flow in the Couette geometry and the foot structure of droplets that is important for heat transport. Of particular interest was the novel initiative of scanning transmission probes of surface heterogeneities that was proposed for the second beamline. In addition, fluorescent nanoprobe elemental mapping techniques and more traditional surface scattering from liquid interfaces were considered in the studies of this class of problems. The novel use of small beams to investigate liquid interfaces within microtensiometers was discussed at length. This interfacial geometry is optimized to probe interfacial dynamics, such as the variation of interfacial structure with tensile/compressive stress.

In summary, the proposed innovative techniques for the second beamline caught the imagination of many participants in the workshop. Novel science was proposed in a variety of scientific areas that involve structural characterization of processes at liquid interfaces. On the basis of this workshop, it appears that NSF’s ChemMatCARS will have a strong scientific community to take advantage of new capabilities to study liquid interfaces on the second beamline.