Role of particle aggregation on the structure of dried colloidal silica layers

TL;DR

This study investigates how drying rate, particle size, and dispersity influence the nanostructure and porosity of dried colloidal silica layers, revealing that particle aggregation primarily determines the packing structure.

Contribution

It demonstrates that particle aggregation and its timescale relative to evaporation govern the layer's nanostructure, challenging the traditional focus on diffusion and convection.

Findings

Faster drying yields denser layers across all suspensions.

Crystalline arrangements occur at lower dispersity and are favored by higher drying rates.

Aggregation over evaporation timescale is key to packing structure, not Brownian diffusion versus convection.

Abstract

The process of colloidal drying gives way to particle self-assembly in numerous elds including photonics or biotechnology. Yet, the mechanisms and conditions driving the nal particle arrangement in dry colloidal layers remain elusive. Here, we examine how the drying rate selects the nanostructure of thick dried layers in four dierent suspensions of silica nanospheres. Depending on particle size and dispersity, either an amorphous arrangement, a crystalline arrangement, or a rate-dependent amorphous-to-crystalline transition occurs at the drying surface. Amorphous arrangements are observed in the two most polydisperse suspensions while crystallinity occurs when dispersity is lower. Counter-intuitively in the latter case, a higher drying rate favors ordering of the particles. To complement these measurements and to take stock of the bulk properties of the layer, tests on the layer porosity were undertaken. For all suspensions studied herein, faster drying yields denser dry layers. Crystalline surface arrangement implies large bulk volume fraction ($\sim$ 0.65) whereas amorphous arrangements can be observed in layers with either low (down to $\sim$ 0.53) or high ($\sim$ 0.65) volume fraction. Lastly, we demonstrate via targeted additional experiments and SAXS measurements, that the packing structure of the layers is mainly driven by the formation of aggregates and their subsequent packing, and not by the competition between Brownian diusion and convection. This highlights that a second dimensionless ratio in addition to the Peclet number should be taken into account, namely the aggregation over evaporation timescale.