# Colloidal self-assembly at an interface

Edited by Qichao Hu

September 19th, 2010

reference: [1]

It is well known that colloidal particles can form at the interfaces between liquids. This phenomenon can be used to self-assemble colloidal particles and ultimately to synthesize new materials.

The colloidal particles' ability to bind to liquid and stabilize emulsion is guided by the need to minimize interfacial energy. When a particle moves from a liquid to a liquid-liquid interface, the change in free energy is

$\Delta G=-\pi R^2\gamma_{OW} (1-\cos\theta_C)^2$

where $R$ is the particle radius, $\gamma_{OW}$ is the energy per unit area of the liquid-liquid interface, and $\theta_C$ is the contact angle. The contact angle is related to the energy per unit area of the various interfaces through: $\cos\theta_C=(\gamma_{PO}-\gamma_{PW})/\gamma_{OW}$, where $\gamma_{PO}$ is at the particle-oil interface, and $\gamma_{PW}$ is at the particle-water interface.

The paper focuses on how to control the assembly of colloidal particles at liquid-liquid interfaces. The control methods are divided into two categories: controlling the geometry at the interface, and controlling the particle-particle interaction at the interface.

1) Controlling geometry at the interface

1.1) droplets as templates

When introducing a liquid into another liquid, the droplets can be used as templates for the colloidal particle formation. Although the particles need to be treated such that they readily absorb at the surface, and sometimes coagulants are added to create rigid structures.

The following image shows octanol droplets in water, and the formation of latex particles on the droplet surfaces.

1.2) flat interfaces

One challenge with the self-assembling colloidal particles at the interfaces is transferring them to a solid substrate. A solution is the dip-coating technique. This relies on the onset of capillary force during the evaporation of the liquid.

One such example is shown in the figure below, where the oil-water interface is lowered to confine the particles between the interface and a solid glass substrate.

As the interface approaches the particles, the particles still remain in the water, since the interface deforms under disjoining pressure. This interface deformation leads to capillary force, which aligns the particles into orderly structures.

1.3) interface between bicontinuous fluids

In addition to having two immiscible liquids such as oil and water, another way to create interface is through phase separation. One example is to rapidly cool a homogeneous mixture. The resulting decomposition leads to phase separation and bicontinuous domains, which under gravity and interfacial energy minimization principle, grow into bulk phases.

The following image shows a water-lutidine mixture with silica particles. We observe phase separation after cooling.

The particle networks formed during the cooling and phase separation is preserved. And adjusting the particle size and concentration, the shear modulus can be tuned over a wide range. The advantage of this system is that the two fluids involved in the phase separation can flow simultaneously and independently even if it is a gel overall.

2) Controlling properties of particles

2.1) Particle shape and size

While small spherical colloids can penetrate and stick onto an interface without distorting it, non-spherical or larger spherical colloids must distort the interface to maintain constant contact angle. For example, in the case of ellipsoidal particles, coatings can be applied to change the contact angle and thus the pattern of aggregation. The principle is to minimize the interfacial distortion.

2.2) Wettability

Introducing surfactants to the particles to change the wettability or hydrophilicity can have dramatic influence on the interaction between the particles and the liquid-liquid interface.

In the following image, when electrolyte (ionic surfactants) is added to the water, the particles at the oil-water interface for crystals. However, when the electrolyte concentration is further increased, the size of the aggregate crystals decreases. This is due to the change in the wettability and in the oil-water surface tension, which affect the capillary force between the aggregate crystals.

There are still many problems about the interaction between particles at liquid-liquid interface that we need to understand, for example in the image below.

The advantage is that colloidal particles can be resolved and tracked and dyed using optical microscopy, unlike atoms or molecules. This gives us a window on the fundamental mechanisms of self-assembly and phase transitions.