2024 Julian C. Smith Lectures

Defect propelled swimming and interactions of nematic colloids for microrobotics

Monday, April 22, 2024 / 4:30 p.m. / 155 Olin Hall

Nematic liquid crystals (NLCs) are highly non-linear fluids that have elastic responses that resist nematogen rearrangement and high-energy defect sites at which nematogen order is lost. Generally, the field of nematic colloids seeks to develop control over these elastic responses and defect structures to tailor colloidal interactions. We have been studying ferromagnetic disk colloids rotated by an in-plane magnetic field in nematic liquid crystals. The disk diameter and rotation rate are sufficiently slow that colloid inertia is negligible. In Newtonian fluids, these colloids rotate without translation. However, in NLC, the colloids’ anisotropic defect structure and the NLC’s elastic response generate broken symmetries that propel colloid translation. For patchy, rough colloids, a defect loop which forms on the disk undergoes periodic defect pinning, release, and contraction. This periodic defect motion generates a swim stroke that powers colloidal swimming. Changes in defect configuration with rotation rate provide a steering mechanism. In addition to this swimming motion, colloid shape and surface chemistry generate long-ranged emergent interactions with neighboring passive colloids in quasi-static settings. Furthermore, the non-linear response of the nematic fluid host allows pair interactions among rotating disks that differ strikingly in range and form from their static counterparts. These interactions provide a rich toolkit for reconfigurable materials assembly and open important fundamental questions regarding swimming at low Reynolds number in NLC.

Peptide surfactants (PEPS) for the Green Separation of Rare Earth Elements

Tuesday, April 23, 2024 / 4:30 p.m. / 155 Olin Hall

We have been developing functional peptide surfactant (PEPS) to meet an urgent societal need. Rare earth elements (REEs) are crucial to modern technologies. These elements are notoriously difficult to separate from each other owing to the similar diameters of the REE cations and the fact that they are typically present in the +3-oxidation state. They are currently commonly separated via liquid-liquid extraction in which oil-soluble extractants complex with the cations at aqueous/oil interfaces and pull them into the organic phase. These LLE processes are poorly selective and require multiple stages to isolate cations with the requisite purity.

We are developing an environmentally friendly REE separation process which exploits PEPS that bind selectively to REEs to form PEPS:REE complexes that adsorb to the air-water interface for recovery via a froth flotation process. PEPS are ‘green’ molecules amenable to design for REE selectivity, interfacial activity, and scalable production. The success of this approach requires that PEPS’ ability to bind selectively to REE cations is retained in the highly anisotropic environment of the fluid interface. As an initial PEPS structure, we have studied a known surface-active lanthanide binding tag peptide. This peptide was designed to coordinate via multidentate interactions with REE cations in a binding loop inspired by the highly conserved EF-hand binding sequence in calcium binding proteins. Using a variety of surface characterization and molecular simulation methods, we show that PEPS:REE complexes are surface active and adsorb with intact binding loops. By rational variation of the initial PEPS sequence, we design PEPS:REE complexes that form monolayers with 1:1 ratios of REE and PEPS, essential to the success of the envisioned separation process. We further show that PEPS can bind and adsorb at fluid interfaces with selectivity among selected pairs of REE cations.

Ongoing work focuses on PEPS with sequences designed for strong selectivity among neighboring lanthanides, on preserving bulk selectivity at the interface, and on the design of foams to recover and re-use these functional molecules. This work is performed by a team of researchers spanning four institutions supported by Basic Energy Sciences at the Department of Energy grant number DE-SC0022240.