Colloids

What is a colloid? A colloid is a suspension of microscopic particles dispersed throughout another substance. Some colloids are translucent due to the scattering of light by the Tyndall effect. Other colloids may be opaque or have a slight colour to them. Some examples of colloids include shaving foams, hair gel, shampoo, tooth paste, mist and fog, milk and mayonnaise.

Shaving Foam

Shaving Foam

Shampoo

Shampoo

Shampoo

Shampoo

Shampoo with glitter particles

Shampoo with glitter particles

Fluid Dynamics

Laminar Flow Dynamics

In the upper part of the image, the yellow-white/pink banding is a result of variations in the density of the medium. These variations arose because of convection currents taking place in the viscous medium as the sample sat on a hot plate. The sample was cooled quickly and so the currents had not time to dissipate. Their presence manifest themselves as variations in the density and thus optical path length through the sample. Streamlines can be seen flowing around the bubbles as they experience a pressure difference which causes them to move in a direction away from the growing crystals.

Invisible Made Visible

Fluid dynamics is the branch of physics describing the flow of liquids. The movement of liquid can be categorised as laminar or turbulent and the magnitude of that flow can be quantified by the Reynolds number. In this case, the fluid flowing around, and in the wake of the bubbles is smooth and laminar. This of course makes sense because the fluid here is a viscous ‘gloup’ of molten sucrose or table sugar and the bubbles move relatively slowly through it. Otherwise invisible, the flow is made visible here through a technique called differential interference contrast.

Shear Flow Vortices

Fluid streams past a solid boundary from left to right. As it does so it experiences friction with the boundary resulting in the application of a shear stress. According to the no-slip condition, the speed of the flow at the boundary is equal to zero, but the speed increases from 0 to v (v = the speed of the fluid) at some height above the boundary. The region between the boundary and the this height is called the boundary layer and it is in this layer that we can see the formation of counter-rotating vortices in the flow. Magnification x100

Vortex in Fluid Flow

In fluid dynamics, a vortex is a swirling or whirling motion of a fluid, which can be observed in a variety of natural and artificial phenomena, such as weather patterns, ocean currents, and even bathtub drains. Here, we can see a vortex occuring along a liquid/air boundary. A vortex can be described as a region of fluid where the flow has a rotational or circular motion around a central axis or point. Vortices are characterized by several key properties, including their size, strength, and persistence. The size of a vortex can range from tiny eddies in a stream to massive storms like hurricanes and tornadoes. The strength of a vortex is determined by the speed and energy of the fluid flow, and is often measured by the vorticity, which is the curl of the velocity vector field. The persistence of a vortex depends on a number of factors, including the initial conditions of the flow, the nature of the surrounding fluid, and the presence of any external forces or perturbations.

Fluid Dynamics

Liquid Crystal

Photographing liquid crystals under a microscope can be a mesmerizing experience due to their distinct physical and optical properties. One of the reasons they are fascinating to photograph is because of their anisotropy, which means that the exhibit birefringence colours when viewed through polarised light. When polarised light enters a liquid crystal, it splits into two rays, each polarized with the vibration directions oriented at right angles to one another, whilst each ray travels at a different speed. Under a microscope with polarized light, this property can result in beautiful, vibrant displays of colour.

What's more, liquid crystals can change phase in response to changes in temperature, as seen in the images below. What is happening here is that the liquid crystal is heated to the phase temperature and as the temperature oscillates around this point, the liquid crystal oscillates between the isotropic and the anisotropic state. Observing these phase changes in real-time is fascinating to watch under the microscope.

Natural Pattern Formations

Branching Creatine

A close-up of Creatine magnified approximately 150 times. Initially, forming a heterogenous mixture, minute crystals of creatine were suspended in a solvent of water. The crystals, jittering about in the molecular storm of Brownian motion, undergo random walks, colliding and adhering to other crystals. Over time, these clusters increase in size, eventually forming the branched fractal structure seen here, called a Brownian Tree. This self-assembly process is called diffusion-limited-aggregation (DLA). The colours in the image are a result of thin film interference. Residual water fills the spaces between the branches. Surface tension causes the top surface of the water to form a meniscus along the branches. As such, close to the branches there is a sudden change in thickness of the fluid film which leads to a change in the wavelength (colour). Regions of the same colour correspond to equal film thickness.

Ammonium Nitrate

Viscous fingering reveals its presence in this image of Ammonium nitrate, magnified approximately 400 times.

A Closeup of Soap!

What is it you might ask? This is a close-up of those colours you can sometimes see on soap bubbles, when the lighting conditions are right that is. Magnified here approximately 250 times, the mezmerising colours come about due a physical process called thin film interference. The process of thin film interference occurs when incident light reflects off the top and bottom surfaces of the a film, in this case the skin of a soap bubble. In order for this kind of interference to occur, the thickness of the film must lie in the nanometer regime. Water gushing around inside the bilayer causes the colours to swirl and the patterns to evolve with time. Where the film is extremely thin, it appears as black as the cosmic abyss.