Decoding the sol–gel saga: How smectite droplets dry

• Smectite clays are minerals that can absorb water to form colloidal suspensions.
• These suspensions can adopt a liquid-like (sol) or a solid-like (gel) state, which determines how their droplets dry.
• Controlling the state of a suspension has however proved to be extremely challenging.
• Professor Hiroshi Kimura at Gifu University, Japan, shows how different physical parameters can be used to fine-tune the drying dynamics and tailor it for practical applications.

Smectite clays are special types of minerals that can soak up water and expand, like sponges absorbing water. Owing to this property, when smectite clays are mixed with water, they separate into tiny solid particles which disperse throughout the liquid without dissolving. Unlike normal solutions, like those obtained, for instance, by mixing small amounts of common salt with water, the clay mineral particles forming a dispersion are large enough to be seen under a microscope and will eventually settle over time if left undisturbed.

Smectite clay dispersions are valuable for their ability to swell, control viscosity, and stabilise mixtures. They enhance drilling fluids, paints, and coatings, trap contaminants in wastewater, improve soil, and act as stabilisers in cosmetics and food. Their high surface area and charge properties also make them useful in pharmaceuticals and environmental cleanup, making them essential across many industries.

The magic of dispersions

Smectite clay dispersions can shift between a sol (liquid-like) and a gel (solid-like) state, making them incredibly versatile. In the sol state, clay particles stay evenly spread in water, creating a smooth, free-flowing mixture. But when conditions change ― such as when more clay is added to the mixture or certain salts are dissolved in the water ― the particles link together, forming a gel with a thicker, more structured texture. This transformation is reversible, allowing these dispersions to adapt as needed.

How droplets dry

Professor Hiroshi Kimura at Gifu University, Japan, carries out cutting-edge experimental work devoted to understanding how droplets of clay dispersions dry and how the state of the dispersion influences this process. He has observed that, when droplets dry, the drying pattern depends on whether the suspension is in the sol or gel state.

By leveraging the sol–gel transition, we can design new materials with tailored properties.

In the sol state, the clay particles move towards the edge of the droplet due to internal convection, forming a ring-like pattern. In the gel state, the clay particles remain in place while water evaporates from the droplet surface, creating a bump-like pattern due to the initial droplet shape.

If the gel-state droplet is placed horizontally before drying, it can form a flat, thin film. Intriguingly, droplets of dispersions close to a sol–gel transition exhibit hybrid patterns with ring-like features and swollen patterns.

Insight into the sol–gel shuffle

A broad understanding of dispersion drying dynamics and of potential means to control it provide critical insights that can impact important applications in engineering, including coatings, functional films, and sensors. Kimura’s work has been devoted to unveiling the role of the different physical factors influencing sol–gel transitions in these materials. He has studied four types of smectite clays — saponite, hectorite, stevensite, and fluorine-modified hectorite — and he has analysed systematically how various factors, including changes in clay particle size and in the concentration of water-dissolved salts, affect the state of the dispersion.

The electrically induced rapid separation effect can significantly speed up particle separation compared to natural sedimentation.

Kimura has shown that larger particles and higher salt concentrations extend the range in which the gel state is dominant. This, in turn, significantly influences the drying mechanism.

He has also demonstrated that the drying behaviour of guest particles or dyes, like Rhodamine 6G or Chinese black ink, in smectite clay dispersions is also dictated by the transition between sol and gel states. ‘By leveraging the sol–gel transition’, explains Kimura, ‘we can design new materials with tailored properties.’

Electrical double layers

In addition to providing robust guidelines for controlling the drying behaviour of smectite clay suspensions, Kimura’s work has deepened our understanding of the physical factors that promote or hinder particle association and that explain the preference of a suspension to adopt a sol or a gel state. When suspended in water, microscopic clay particles interact with dissolved ions (charged atoms), attracting them to their surface and binding them tightly. An outer layer of more mobile ions also forms on top of the inner layer. The presence of a double layer of charged ions induces repulsive electrostatic forces, which tend to separate the suspended clay particles and promote their mobility, decreasing the fluid’s viscosity.

The electrorheological effect

When ions are removed from water, a process known as deionisation, the double layer expands, preventing particle aggregation. In the case of smectite clay suspensions, deionisation promotes better dispersion of the suspended particles, driving the transition from the gel to the liquid-like sol state. Kimura has been the first in the world to show that the sol state of smectite clay dispersions exhibits the so-called electrorheological effect, a measurable and often dramatic increase in viscosity occurring when an electric field is applied to them. In smectite clay suspensions, this increase in viscosity effectively causes a transition from a liquid-like to a solid-like behaviour, in which individual clay particles are prevented from flowing freely by the interactions with other particles.

Electrically driven clay separation

Kimura has extended further his work on smectite clay suspensions in the sol state by examining their behaviour when subject to an electrical current. Kimura has shown that the clay particles form flocs under a direct current (DC) field. This effect has also been observed in various colloidal dispersions.

This phenomenon, which Kimura named the electrically induced rapid separation (ERS) effect, can significantly speed up particle separation compared to natural sedimentation. In deionized clay dispersions, the ERS effect results in increased viscosity and a more elastic nature. The reason why the flocs do not settle is likely due to their extremely small size. This has important implications for water purification processes, industrial waste treatment, and the development of smart fluids, where electric fields can be used to control the state of a dispersion dynamically.