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1. Phase transitions in colloids under microgravity

   Lei, Qian ; Khusid, Boris ; Kondic, Lou ; Hollingsworth, Andrew D. ; Chaikin, Paul M. ; Meyer, William V. ; Reich, Alton J. ( March 2021 , American Physical Society March Meeting 2021, March 15-19, 2021, Virtual)

   null (Ed.)

   Research on colloids is motivated by several factors. They can be used to answer fundamental questions related to the assembly of materials, and they have many potential applications in electronics, photonics, and life sciences. However, the rich variety of colloidal structures observed on the Earth can be influenced by the effects of gravity, which leads to particles settling and the motion of the surrounding fluid. To suppress the gravity effects, experiments on concentrated colloids of spherical and ellipsoidal fluorescent particles were carried out aboard the International Space Station. The particles were suspended in a decalin/tetralin mixture to match the particle refractive index. Confocal microscopy was used to visualize the particle behavior. The work was supported by the NSF CBET grants 1832260 and 1832291 and the NASA grant 80NSSC19K1655. 

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2. Writing in the granular gel medium

   https://doi.org/10.1126/sciadv.1500655  

   Bhattacharjee, Tapomoy ; Zehnder, Steven M. ; Rowe, Kyle G. ; Jain, Suhani ; Nixon, Ryan M. ; Sawyer, W. Gregory ; Angelini, Thomas E. ( September 2015 , Science Advances)

   Gels made from soft microscale particles smoothly transition between the fluid and solid states, making them an ideal medium in which to create macroscopic structures with microscopic precision. While tracing out spatial paths with an injection tip, the granular gel fluidizes at the point of injection and then rapidly solidifies, trapping injected material in place. This physical approach to creating three-dimensional (3D) structures negates the effects of surface tension, gravity, and particle diffusion, allowing a limitless breadth of materials to be written. With this method, we used silicones, hydrogels, colloids, and living cells to create complex large aspect ratio 3D objects, thin closed shells, and hierarchically branched tubular networks. We crosslinked polymeric materials and removed them from the granular gel, whereas uncrosslinked particulate systems were left supported within the medium for long times. This approach can be immediately used in diverse areas, contributing to tissue engineering, flexible electronics, particle engineering, smart materials, and encapsulation technologies. 

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3. Electric field driven aggregation of negatively and positively polarized particles in dilute suspensions.

   Khusid B., Lei Q. ( March 2020 , Bulletin of the American Physical Society)

   A variety of colloidal structures observed in terrestrial experiments could also have been influenced by gravity effects (particle sedimentation, convection, etc.) It is often assumed that weightlessness simulated in a time-averaged sense by slowly rotating a specimen in a clinostat about an axis perpendicular to the gravity direction that is widely used in biological tests would reduce the effect of gravity on suspensions. Experiments on a non-buoyancy-matched suspension in flights in NASA Zero-gravity aircraft revealed that particle patterns formed in a clinostat and under normal gravity are actually similar. A requirement for matching densities between particles and a solvent severely limits possibilities to study the field-induced structuring in colloids in terrestrial experiments. Long-term microgravity in ISS offers unique opportunity to employ not density matched suspensions to explore a wide range of the mismatch of electric characteristics between particles and a solvent. We will report experimental data on the field driven structure formation in suspensions and present our approach to the development of ISS experiments. The aim is to understand mechanisms of structure formation and suggest novel routes for creating functional materials. *NASA NNX13AQ53G, NSF1832260. 

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4. Formation of colloidal chains and driven clusters with optical binding

   https://doi.org/10.1039/d2sm00393g  

   Davenport, Dominique J. ; Kleckner, Dustin ( June 2022 , Soft Matter)

   We study the effects of the optical binding force on wavelength sized colloidal particles free to move in a counter-propagating beam. This work is motivated by the concept of using optical binding to direct the assembly of large numbers of colloidal particles; previous work has used small numbers of particles and/or 1D or 2D restricted geometries. Utilizing a novel experimental scheme, we describe the general static and dynamic self-organization behaviors for 20–100 particles free to move in 3-dimensional space. We observe the self-organization of the colloids into large optically bound structures along with the formation of driven particle clusters. Furthermore we show that the structure and behavior of these optically bound systems can be tuned using the refractive index of the particles and properties of the binding light. In particular, we show that the driven behavior originates from N -body interactions, which has significant implications for future work on optically bound clusters of more than 2 particles. 

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5. Model hard ellipsoids: the practical matter of producing them.

   Hollingsworth, A. ; Chaikin, P.M. ; Kondic, L. ; Reich, A. ; Khusid, B. ( March 2020 , Bulletin of the American Physical Society)

   Model hard colloids have a great deal of relevance to physics and in particular the study of their phase behavior which can mimic that of simple atomic liquids and solids. "Nearly hard colloidal sphere" suspensions were formulated 35 years ago by the Ottewill group (Univ. of Bristol) and Imperial Chemical Industries Ltd., which were used by Pusey and van Megen in their seminal study of the phase behavior of hard-sphere colloids. We report on our efforts to reproduce and refine this benchmark polymer colloid, including the recent synthesis of hard ellipsoids for random and ordered packing studies in microgravity*. The custom-made samples are composed of linear polymer chains of poly(methyl methacrylate), functionalized with photo-crosslinkable moieties and fluorescent molecules. The resulting ellipsoidal shapes are about 1 micron in size and stabilized with surface-grafted poly(12-hydroxystearic acid) chains. The particles are dispersed in a refractive index matching fluid and particle aspect ratios vary from 1 to 4. * Launched March 2020 aboard SpaceX CRS-20 resupply service mission to the International Space Station. *NASA NNX13AR67G (NYU); NSF GOALI 1832291 (NYU); NSF GOALI 1832260 (NJIT) 

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