Understanding the Formation of Anisometric Supraparticles: A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface

Abstract

Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop-air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation. This leads to a buildup of a stiff fumed silica mantle of ∼20 μm thickness that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles.

Droplets, Evaporation and a Superhydrophobic Surface: Simple Tools for Guiding Colloidal Particles into Complex Materials

“…Another aspect attributing quite some elegance to this method is the fact that the application of this layer can occur on any surface able to resist the temperatures used for the calcination step. Moreover, if carefully done, this surface also provides high transparency [51], which for example even allows experiments with an inverse optical microscope setup for observing the inside of the droplet from below the surface [60]. It might be noted here that the curvature of small-scale roughness was revealed to play a key role for achieving high resistance against wetting, thus being of major importance for acquiring superamphiphobicity [61].…”

“…Large particles contained within the drying solution similarly undergo surface collection in analogy to polymers [28,60]. In the case of the doughnut silica supraparticles, the colloidal particles (330-nm diameter silica microspheres) get collected at the interface of the precursor droplets due to the shrinking surface, which is propagating faster towards the interior than can be counterbalanced by particle diffusion [92].…”

“…By labeling the FS and the water phase with different fluorescent dyes, the density profiles of the FS perpendicular to the droplet surface, i.e., in the radial direction, could be deduced throughout the drying process. Thereby, it was shown that the FS particles accumulate at the droplet surface and that the deformation of the droplet takes place once a certain effective thickness and density of this shell is reached [60]. In a somewhat related study, the evaporation process of an aqueous lysozyme solution on a superhydrophobic PMMA surface has been followed by microbeam SAXS.…”

“…The resulting supraparticle anisometry, i.e., the ratio of principal axes lengths, could be precisely tuned within the range of 0.5–25 mM of ionic strength, and the observed anisometry is directly proportional to the logarithm of the ionic strength and reaches values of up to 1.6 [93,96]. A closer look into the formation mechanism revealed that the deformation occurs at a constant surface excess concentration of FS [95] and for an effective shell-thickness of about 22 µm, which corresponds to an interfacial FS volume fraction of 0.17 [60]. It might be noted that shell formation during evaporation can be related to buckling of the drying droplets.…”

Evaporation and dissolution of droplets in ternary systems

“…Supraparticles (SPs) refer to three-dimensional macroscopic structures by selfassembly of colloidal (micro-) nanoparticles [163][164][165][166]. Such particles have been identified as prominent candidates for a wide variety of modern applications like catalysis [167,168], catalytically active particles [169], adsorbents in environmental pollution management [170,171], diagnostics [172], chromatography [173], photonics [163,174], barcodes [175], biomedical delivery [172], and sensing [176,177].…”