Having rather skipped over what all that really means in the earlier columns, I thought I would return to the scientific research side of the CRC and explain a bit more of the instruments and techniques Ray and his colleagues, in the Research School of Physical Sciences and Engineering at the Australian National University, are using.
It is of great value in understanding why things happen in practice, to know a bit about the science underlying what we can see. As noted nearly two years back, we see water and ink soak into a sheet of paper and assume that they take a simple path downward via any holes or cracks in the surface of the sheet, which sounds perfectly logical, but that assumption fails to explain why dots spread after printing or why the penetration rate is often slower than expected.
Whilst observation is often a great tool for working out how things happen, what occurs when a blob of ink or fountain solution hits a piece of paper is not easy to measure, as what is being studied is continuously changing.
An ingenious idea that Rayhas perfected developed is to freeze, at very low temperature, a sample of paper immediately after a droplet of liquid has been placed on it to stop any movement, and thus “preserve” an effect until there is a chance to study it. The medium for the freezing process is nitrogen slush, which is much colder than liquid nitrogen which boils at minus 195 degrees Celsius, and if that’s not enough to freeze any sample, I’d like to know why.
The process of working with very low temperature materials is called cryogenics; from the Greek Kryos – meaning icy cold and is not the same as cryonics; which is the process of preserving bodies until science invents a way they can be thawed out and revitalised. (Incidentally there is no evidence to support the urban myth that Walt Disney was ever frozen).
Once a sample has been frozen at such a low temperature, it is maintained in that state in a specially developed cold stage of the instrument that is used to take the pictures; the scanning electron microscope.
The term electron microscope has been around since the thirties, when the first transmission instrument was developed. About 10 years later the first scanning electron microscope (SEM) came into being.
The need for the electron microscope was generated when scientists wanted to look at very tiny samples, as the optical microscope with which we are all familiar with cannot observe things that are smaller than the wavelength of light, due to diffraction.
The effect called diffraction occurs when electromagnetic radiation (of which visible light is but one band sandwiched between infra red and ultra violet) is bent as it passes the edge of an object (a good example is the rainbow pattern that appears on an oil slick). Diffraction gets worse as the sample being looked at gets closer to the wavelength that is looking at it, down to a point where resolution becomes impossible.
Optical microscopes with magnifying lenses were invented and developed in Europe in the 17th Century, with some famous names including Galileo and Robert Hooke (and some not so famous ones like Hans Lippershey, Hans and Zacharias Janssen, Eustachio Divini, and Antonie van Leeuwenhoek) being a part of the development process. An SEM vaguely resembles a common microscope as it has an illumination source, a tube down which the energy passes and a means of collecting whatever is reflected back, but that’s about it.
The electron source of the SEM comprises an electron generator, which is a super-muscle variant of the electron gun in a television set or CRT monitor. A simple version comprises a tungsten wire, which acts as a cathode from where electrons are boiled off (at about 2,7000C) and then attracted/accelerated toward an anode by a large voltage potential difference. An electron accelerated across a potential difference of one volt is called an electron volt (eV), and a thousand electron volts designated as a KeV. In big machines very powerful electrons, of up to 10,000 KeV are generated. (The limit of magnification with visible light is less than 1000 times, whereas the SEM can magnify up around a million times, as electrons are so very much smaller than the wavelength of visible light. For example; a 10,000 KeV electron has a wavelength about one fifty thousandth that of visible light).
Once generated, the electrons are focused by a series of electromagnetic “enses” and directed at the sample to be measured. All of this action occurs under vacuum. As the name implies, a scanning microscope scans the lines of electrons back and forth (a bit like the lines on a TV screen). As the electrons strike the sample surface they cause quite a deal of activity. Some are bounced off (secondary electrons), some are absorbed into the sample then radiated off in all directions (these are called backscattered electrons) and some generate X-rays. The effects of all of these can be collected and measured.
In order for the sample to be seen it must be conductive and the standard means of ensuring that this is so, is literally to gold plate it. The gold coating is extremely thin, and is applied in a vacuum chamber. In Ray’s work, the coating chamber had to be kept at those low freezing levels in order to maintain the sample in suspended animation.
The secondary electrons are caught on a device called a scintillator that turns their arrival into flashes of light that are in turn amplified and displayed on a monitor. These pictures give rise to those images of gnat’s noses that we have all seen, whilst the backscattered electrons can give an idea of what goes on inside the sample, including the presence of various elements.
In order to better track the passage of the liquid through paper fibres, Ray added a tiny amount of a caesium salt (which is an element with an atomic number of 55, and one of only three metals that are liquid at room temperature, the others being gallium and of course mercury), as this addition assisted in the generation of clear images.
As noted earlier, Ray’s research clearly proved that when applied to a piece of paper, a liquid like water or ink does not flow through pores into the paper, but runs along channels formed by fibres lying beside each other in the form of films which thicken and then fill the pores. When a fibre has been sized to increase its water resistance (refer to December 2002 and June 2003 articles), the fluid travel is just below the surface of the fibre, but still in the form of film flow, not as the textbooks have been saying for decades.
This information is now being studied by paper manufactures with an eye to increasing their degree of control of the papermaking process, and the offshoot of that will be substrates that meet the ever-growing demands of the industry with regard to new and high speed printing techniques and applications.