Colloidal particles in drying liquid films

We have active experimental and modelling studies of colloids, especially in the area of drying thin films of colloidal suspensions - this is how many coatings and paints are made.

Colloids, consisting of particles in a fluid, are used to deposit layers for a wide range of applications, such as adhesives, inks, pharmaceuticals, agricultural treatments, cosmetics and paints. The Group has more than two decades of experience in studying the multi-step process by which colloidal polymer particles in water create a dry polymer film. This process, which is called latex film formation, is illustrated below.

When water evaporates, the average distance between particles decreases, until the particles are close packed. Under the right conditions, mono-sized particles will pack into a crystalline (typically face-centred cubic, or FCC, structure. A reduction in the surface energy in combination with capillary forces generated by the water menisci in between the particles provide a driving force for the spherical particles to be deformed.

If the particles are “soft” enough, they will eventually fill all available space. When the voids between the particles disappear, light is no longer scattered from the interfaces, and the optical clarity of the film increases. Cross-sections of a latex film sometimes show the hexagonal array of particles in the (111) plane deformed to create a honeycomb pattern. Over time, polymer chains diffuse across the particle boundaries and a uniform film results.

Confocal microscope image plus simulation snapshot of stratified films

Above and on the left is a confocal laser scanning microscopy of a dried film formed from a binary mixture of large particles and small pH-responsive particles. Large particles are labeled with a red fluorescent dye, and small pH-responsive particles are labeled with a green dye. Note the green layer on the top of the dried film: the mixture of small and large particles have spontaneously segregated during drying.

Above and on the right are a pair of computer simulation snaphots, of a simulation study of drying of an initially liquid film that contains large (red) and small (gold) particles (solvent is not shown). The left-hand snapshot shows the initial film, where the large and small particles are well mixed. The right-hand snapshot shows the final dry film, where a layer of small (gold) particles has formed on top of a layer of mostly large (red) particles. This is an example of a spontaneous out-of-equilibrium self-organisation process.

Polymer-in-water dispersion