Ever since mayonnaise was invented in 1756, people have wanted to make more soft materials. From the moisturising cream that you rub into your face, to the paint that you spread on your walls, soft materials are all around us.
What makes a soft material soft? You can consider anything that starts to flow when you apply some force to it (but otherwise doesn’t) as a soft material. Think about toothpaste. Only when you squeeze the tube does it flow. Contrast that with a coffee table, which hopefully does not flow under any circumstances, or a cup of coffee, which spills at the first available opportunity. Soft materials are the materials most relevant to life itself: gels, pastes, creams, blood, food etc.
One common type of soft material is an emulsion. Emulsions consist of tiny droplets of one fluid mixed in with another fluid. Usually, these droplets merge with other droplets to make a bigger droplet, eventually causing the fluids to separate: imagine a salad dressing before shaking. But if you add tiny particles to the fluids – particles that are 100x smaller than a human egg – they adsorb at the interface of these droplets and stop them from banging into each other. That’s how you make a stable emulsion like mayonnaise.
But how do we make new soft materials? One way is to control the way these particles assemble and interact with each other at the interface. If you can control the structures and patterns they form, you might be able to control other properties of the emulsions, such as their viscosity.
Spooky Action at the Interface
Once particles adsorb at fluid-fluid interfaces, they can act as if they have a mind of their own: spherical particles often attract each other from far away and clump together, and non-spherical particles can do the same but also align with each other in preferred orientations, too.
The reason they do this is because of interface deformations. Particles adsorbed at an interface deform it, and these interface deformations can attract and repel each other, just like electric charges.
Interface deformations around particles come in several flavours that physicists call modes. The monopolar mode occurs when a particle is heavy and deforms the interface by causing it to sink, just like a bowling ball on a trampoline. This mode is responsible for the Cheerios effect.
Another mode that scientists have already observed in nature is the quadrupolar mode, which occurs when particles are non-spherical. I’ll spare you the details, but to fulfil something called Young’s equation, the interface has to deform around the particle, even when it is too light to depress the interface like the Cheerios do. This mode is responsible for the beautiful arrangements of mosquito eggs seen on the surfaces of ponds.
But there is another mode, called the dipolar mode, which should theoretically exist but has yet to be realised by scientists. Until now.
Dipolar Capillary Interactions
I, and several colleagues, recently proposed a way to create dipolar capillary interactions between particles at interfaces: by placing non-spherical magnetic particles at an interface and subjecting them to an external magnetic field. (10.1039/C4SM01124D)
The magnetic particle tries to align with the magnetic field, but the surface tension of the interface pulls it back down, and so the particle tilts with respect to the interface. It turns out that when the particle tilts, it deforms the interface in a dipolar fashion.
If we keep increasing the magnetic field strength, eventually the magnetic torque overcomes the surface tension pulling against it and the particle flips discontinuously (scientists call this a first-order phase transition) from a tilted-state to a vertical state, aligned with the field. And in this vertical state there are no interface deformations, and therefore no capillary interactions.
So the dipolar mode that we discovered is drastically different compared with the monopolar and quadrupolar mode that scientists already knew about. In the monopolar and quadrupolar modes, the particle properties alone, namely weight and shape, determine the strength and form of the capillary interactions: once you place the particles on the interface, you have almost no control over the capillary interactions between the particles and therefore the assembly that takes place.
In the dipolar mode that we discovered, the strength of the capillary interactions between the particles can be tuned by controlling the particles’ tilt-angle (the external field controls the tilt-angle); if the tilt-angle is small, the capillary interactions are weak, if the tilt-angle is large, the capillary interactions are strong.
This is significant because the type of structures you achieve – the way the particles arrange on the interface – depends on the capillary interaction strength, as we showed in our paper. (http://onlinelibrary.wiley.com/doi/10.1002/adma.201402419/abstract)
Even better, you can completely switch off the capillary interactions by exceeding a critical magnetic field strength whereby the particles jump from the tilted to the vertical state. Therefore, you have much more control over the assembly process when compared with the monopolar and quadrupolar interactions.
Our work could provide valuable insights for researchers trying to create new soft materials by controlling the assembly of particles at interfaces.