Self-assembly of colloidal particles from evaporating droplets: role of DLVO interactions and proposition of a phase diagram

TL;DR

This study investigates how DLVO interactions and pH influence the shape of colloidal deposits from drying droplets, proposing a phase diagram that explains different deposit patterns through flow competition.

Contribution

It introduces a phase diagram linking deposit shapes to flow patterns and DLVO interactions, validated by experiments and simulations.

Findings

Deposit shape depends on pH and DLVO interactions.

A phase diagram explains transition between ring, bump, and uniform deposits.

Simulations agree well with experimental results.

Abstract

The shape of deposits obtained from drying drops containing colloidal particles matters for technologies such as inkjet printing, microelectronics and bioassay manufacturing. In this work, the formation of deposits during the drying of nanoliter drops containing colloidal particles is investigated experimentally with microscopy and profilometry, and theoretically with an inhouse finite-element code. The system studied involves aqueous drops containing titania nanoparticles evaporating on a glass substrate. Deposit shapes from spotted drops at different pH values are measured using a laser profilometer. Our results show that the pH of the solution influences the dried deposit pattern, which can be ring-like or more uniform. The transition between these patterns is explained by considering how DLVO interactions such as the electrostatic and van der Waals forces modify the particle deposition process. Also a phase diagram is proposed to describe how the shape of a colloidal deposit results from the competition between three flow patterns: a radial flow driven by evaporation at the wetting line, a Marangoni recirculating flow driven by surface tension gradients, and the transport of particles towards the substrate driven by DLVO interactions. This phase diagram explains three types of deposits commonly observed experimentally, such as a peripheral ring, a small central bump, or a uniform layer. Simulations and experiments are found in very good agreement.