Scanning probe microscopy is a method of examination where a physical probe that is effectively dragged across a specimens surface in order to gather mechanical information about it. As more new materials are created and applied in new situations the importance of understanding the rheology (the study of the flow of matter) of these materials cannot be overstated. Often the linear elastic approximation offered by Hooke’s law is vastly insufficient and so methods such as atomic force microscopy, an incredibly high resolution version of scanning probe microscopy, is employed to gather information on how surfaces respond to forces on an atomic scale.
Fluid dynamics is an interesting area of physics to study. In many ways where it shines is the seemingly disparate connections that it can form to other areas of physics. For instance in a 2D plane, the interaction of vortices in the liquid is very similar to the interaction of electric charges. The swirling fluid centres actually attract one another when they have counter-rotating motion and repel when they share co-rotating motion. Some people actually choose to imagine the electromagnetic field spreading out like a spiral from the charged particle (a combination of radial electric field and concentric magnetic field) which makes the vortex interaction the more fundamental system. Another would be the example of vortices (sometimes called maelstroms) in the ocean. These massive swirls move across the Atlantic and models seem to show that the process of water becoming trapped in the vortex for transport can be seen as analogous to that of the trapping of matter by a black hole.
The manipulation of waves is very important in physics. Lenses can be seen as the most basic form of manipulation which change plane waves into focussing waves and vice versa. Beyond this, metamaterials are designed to have a structure on a scale below that of a wavelength in order to gain properties that no natural material would ever possess. For the control of electromagnetic and acoustic waves often the phase of the waves are controlled so that the propagating wave progresses according to a generalised Snell’s law. A new branch of metamaterials are composed of two dimensional structures of sub wavelength cells which allows for the manipulation of waves in a flat geometry rather that volumetric modulation which is the norm.
The science of membranes is something a reader of this blog will be all too familiar with. It’s not too surprising that they are often the feature of scientific research considering that membranes are used in water filtration both for drinking sanitising waste water; pharmaceutical separation of liquids; and in many medical scenarios such as the purification of blood that has been donated. Often the biggest problem with membranes is they have a tendency to clog over time. The clogging with bacteria, biofouling, is one of the most common types and has been talked about in posts here and here. Today however the more general mechanism of how membranes fail will be looked into.
The diagram on the right shows the general structure of a transmission electron microscope. While visible light, with an average wavelength of 500 nm, may never be able to resolve an atom 1 nm wide an electron actually has a wavelength inversely proportional to its momentum. This means that accelerating an electron and giving it more energy will let it interact and image smaller and smaller things. After the acceleration fields (which would exist between the condenser lens and the specimen) the electromagnetic lens act to move the electron beam like a beam of light to eventually produce an image. Now the specimen will always be incredibly thin, less than 100 nm thick, as the sample will invariably slow down the electrons that pass through it which is counter productive to a greater resolution. A thinner specimen reduces this effect and maintains the imaging precision this device is used for.
The reason why we’ll never see an atom through a microscope is quite clear: an atom is about half a nanometre across, while visible light has a wavelength of about 500 nanometre on average. This means that its simply impossible to resolve (distinguish as separate) two atoms, it can’t be done. Photonic crystals are artificial crystal structures designed to control the motion of light passing through them. There are many analogies between a photonic crystal interacting with light and semiconductor crystals interacting with electrons. In particular band theory applies, with only certain wavelengths of light being permitted within the crystal with the crystal’s periodicity creates “band gaps” where groups of modes are forbidden. Wavelengths approaching those blocked by the stop-bands suffer highly directional based dispersion resulting in effects such as negative refraction and self collimation.
In a world of evergrowing tension, international trust is at an all time low. With the recent Russian diplomat expulsions and resulting retaliatory expulsion of mostly US diplomats from Russia, there is a considerable political conflict brewing. This is particularity worrying as currently over 93% of the world’s military nuclear capacity is held between Russia and the United States. Russia currently has about 7000 nuclear missiles and the US has approximately 6800. The rest of the world’s cumulative might barely breaches 1000.