Spotlights pubs.acs.org/JPCL
Spotlights: Volume 8, Issue 19
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SUPPRESSION OF THE COFFEE-RING EFFECT AND EVAPORATION-DRIVEN DISORDER TO ORDER TRANSITION IN COLLOIDAL DROPLETS As you read this, there’s a good chance that you’re holding a cup of coffee, whether it’s first thing in the morning or to counter a midafternoon slump. You may have even spilled a bit on your desk and noticed something interesting about the drop as it dried: The resulting stain is shaped like a ring, with the coffee concentrated on the outside edge. This phenomenon goes unnoticed by most people, but to scientists the “coffee ring effect” can cause some headaches. When a colloidal solution dries, it leaves behind a deposit of the particles sans the solvent. Spontaneous self-assembly of these particles into an ordered structure could have many applications, including fabrication of optoelectronic devices and biosensors, but such applications tend to be compromised by the coffee-ring effect. Efforts have been made to suppress the coffee-ring effect, typically by the application of an electric field, or by using nonspherical particles, or by making the substrate superhydrophobic with special structuring. Das et al. (10.1021/ acs.jpclett.7b01814) developed a simple technique to completely suppress the coffee-ring effect, and they found that it is efficient for any type of substrate (without any structuring or patterning) and for any shape of the particles, including spherical ones. By coating a flat glass substrate with silicone oil, the authors achieved complete suppression of the coffee-ring formation when a droplet of aqueous colloidal suspension deposited on the substrate was evaporated. The authors also studied the dependence of self-assembly leading to well-ordered single-crystalline structures on the parameters that control the drying process. Although they found that a longer drying time leads to better ordering, their results also establish a surprising dependence of the ordering on the size of colloidal particles, with a systematic enhancement of ordering with a decreasing size of particles.
systems under nonequilibrium conditions without the complexity of live intracellular measurements. Similarly, the method is applicable to studying energy-independent self-replicating systems that generate monomers in numbers geometrically proportional to the number of resulting assemblies (e.g., viruses).
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ELECTROLYTE-INDUCED INSTABILITY OF COLLOIDAL DISPERSIONS IN NONPOLAR SOLVENTS That cup of coffee in your hand may seem like a magic serum at times, but it can also be referred to as a colloidal dispersion, with the coffee dispersed throughout the water. Scientists study the attraction and repulsion of colloidal particles to develop a better understanding of the stability of colloidal dispersion, with the goal of controlling it. Generating new ways to mediate colloidal stability and instability would be highly beneficial for controlling the properties of nanoparticles, particularly in nonpolar solvents, where charge numbers are typically low and van der Waals attractions are weak. Existing theories of colloidal stability cannot explain how particles interacting through a charge repulsion over moderate distances could become attractive over very short distances. Smith et al. (10.1021/ acs.jpclett.7b01685) report an interesting interaction between soft matter particles dispersed in electrolyte solutions in low dielectric media. The authors added an oil-soluble electrolyte to otherwise stable dispersions of colloids, destabilizing them and causing them to visibly aggregate and sediment. By optically trapping the colloids, they observed that the interactions were attractive in electrolyte solutions. This finding was surprising, because over moderate distances the colloids repelled each other, as would be expected for charged particles in an electrolyte solution. The authors show that this attraction and instability can only be due to the addition of electrolyte and not due to any specific functionality of the particle or the nonpolar solvent chosen. Existing theories of colloidal stability cannot explain how particles interacting through a charge repulsion over moderate distances could become attractive over very short distances, and future work will be needed for a complete understanding of the effect.
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CREIM: COFFEE RING EFFECT IMAGING MODEL FOR MONITORING PROTEIN SELF-ASSEMBLY IN SITU While work is being done to suppress the coffee-ring effect, some researchers are instead using it to their advantage when studying protein self-assembly. Shaw et al. (10.1021/ acs.jpclett.7b02147) have developed an imaging model that makes use of the coffee-ring effect. They used the radial capillary flow within a drying sessile droplet (the coffee-ring effect) to emulate dynamic native environments, and then they monitored an archetypal protein assembly in situ using highspeed super-resolution imaging. Their coffee-ring effect imaging model (CREIM) showed that protein assembly can be empirically driven to completion within minutes to seconds, versus the hours needed for the same assembly in solution, without apparent changes in supramolecular morphology. These results show that CREIM is well suited to monitor homogeneous protein self-assembly in situ. CREIM also offers an optimal measurement platform to diagnose self-assembling © 2017 American Chemical Society
Published: October 5, 2017 4942
DOI: 10.1021/acs.jpclett.7b02546 J. Phys. Chem. Lett. 2017, 8, 4942−4942
