Steering Colloids via Two-Dimensional Diffusiophoresis Using Crossed Gradients in Salt Concentrations

Colloidal particles in liquids are found in many ordinary items, such as foods, paints, inks, cosmetics, and pharmaceuticals. They also are the "building blocks" for complex materials with attractive optical, mechanical or electrical properties. Colloids in liquid are not static, but they are constantly moving in random directions because of Brownian diffusion. In this research, we will investigate ways to move colloids in water in specific desired directions with exquisite control - thereby defying Brownian motion.

When colloids have an electric charge they can be steered by an external electric field acting upon them through the mechanism known as electrophoresis. Recently, scientists have discovered that the electric fields created by gradients in salt concentration in water can drive colloid motion. The colloidal particles will move linearly either up or down a gradient in salt concentration, depending on the type of salt and whether the charge on the particles is positive or negative.

Our vision in this project is to steer particles on curved paths by putting them in solutions in which there are gradients of two different salts going in directions at right angles to each other. We will use criss-crossed polymeric fibres to release salts into water as a way to devise complex patterns of salt concentrations. According to some recent calculations, it should be possible to use these salt gradients to separate mixtures of particles that differ in their charge. This new concept will allow particles to be sorted in a simple way, and at low-cost and without a need for an external power supply. An immediate application will be in re-using and recycling expensive nanoparticles to minimise their waste. The use of crossed gradients in salt concentration could also provide a way to measure unknown electric charges on particles. When the concept is validated in our experiments, it will provide the basis for a simple diagnostic method to identify and characterise particles, such as viruses or contaminants.

Our research will show that crossed salt gradients can overcome Brownian motion to steer sub-micrometer particles on nearly any desired path. Having such precise control of colloid motion will open up possibilities for fabricating complex materials one particle at a time. We will attach some "sticky" molecules onto the colloids so that they will adhere to surfaces after being steered there by electrophoresis. Our fundamental research might thereby lead to breakthroughs in the manufacturing of materials for applications ranging from solar cells and optical devices to delivering drugs in the body.