Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F ∼ 1/r 2 . Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l = 16 ± 3 nm, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivity.
One
of the most attractive commercial applications of semiconductor
nanocrystals (NCs) is their use in lasers. Thanks to their high quantum
yield, tunable optical properties, photostability, and wet-chemical
processability, NCs have arisen as promising gain materials. Most
of these applications, however, rely on incorporation of NCs in lasing
cavities separately produced using sophisticated fabrication methods
and often difficult to manipulate.
Here, we present whispering gallery mode lasing in supraparticles
(SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing
medium and cavity. Moreover, the synthesis of the SPs, based on an
in-flow microfluidic device, allows precise control of the dimensions
of the SPs, i.e. the size of the cavity, in the micrometer
range with polydispersity as low as several percent. The SPs presented
here show whispering gallery mode resonances with quality factors
up to 320. Whispering gallery mode lasing is evidenced by a clear
threshold behavior, coherent emission, and emission lifetime shortening
due to the stimulation process.
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