On the face of it…

Our ability to measure on the micro- and nano- scale is vital in many everyday items - and if you get the topology correct there is often no need to scratch beneath the surface… Each and every one of us encounters various surfaces, every minute of every day of our lives. Whether these are the pavement, clothes, or computer keys, have you ever wondered what it is that makes them feel the way they do – why certain foods have smooth textures and do not scratch the inside of our mouths, or why make-up feels smooth on our face. The need to view and measure surfaces on the micro- to nano-scale is key to ensuring that surfaces interact with each other, and us, in specific ways. For example, if you run your hand over the bottom of a ski, you will find it smooth to the touch, yet it produces enough friction with snow to enable skiers to manoeuvre in a safe, and fun, manner. One issue when working on such a small scale is that there is the potential for inaccuracies, often as a consequence of the examination limitations of our eyes alone. As such, all surfaces are examined during the production process and as part of various quality control protocols, often using microscopy techniques. Optical metrology therefore has implications across the medical, scientific, automotive, food and leisure industries, to name but a few; and it is of vital importance that all surface topology measurements are as accurate and reproducible as possible to maintain consistency and quality.

In order to be sure that surface measurements occur in a reliable and accurate manner, there are two methods that are widely accepted within industry. As the traditionally employed technique, contact profilometry incorporates the use of a diamond stylus. This is moved vertically, in direct contact with a sample’s surface, and then moved laterally across the surface for a specific distance and with a defined contact force. Surface measurements are therefore obtained due to stylus displacement as a function of position. However, a stylus can be problematic since it cannot be used on certain materials, such as adhesives, and the dragging action can cause damage to the surface itself. As a technique which has been a more recent addition to the field of metrology, confocal laser scanning microscopy (cLSM) is a non-contact optical imaging system for obtaining high resolution images with depth sensitivity. Samples are scanned point by point and reconstructed using specialised computer software, which produces three dimensional images of topologically complex objects. cLSM is therefore a reliable and sensitive method of generating fast, traceable roughness measurements.

cLSM techniques are used in a plethora of laboratories on a regular basis, for a broad spectrum of applications. Many of these affect the day-to-day life of members of the general public. Since surface topology is essentially a study of surface textures, each and every surface poses a potential application.

Chocolate

Quality control plays an extremely important role in the manufacture of any food to ensure that it is safe for consumption. In the production of chocolate, one of the major concerns is with the development of fat bloom. This phenomena arises from compositional changes to the fat (the cocoa butter), causing it to migrate to the surface of the chocolate and re-crystalise. This results in flecks of lighter colour on its exterior and a decrease in the smooth texture and glossy finish commonly associated with chocolate.

It is thought that fat bloom is caused by poor tempering, inefficient cooling methods, warm storage conditions, the interaction of certain fats which are incompatible with cocoa butter, and abrasions. During the manufacturing process, temperature is used to ensure that stable fat crystals form while the chocolate hardens. The presence of fat bloom is therefore a good indicator that the chocolate has not been maintained in an optimal environment, thus decreasing its quality. cLSM has been used to assess the surface roughness of bloomed chocolate, as an indirect measure of quality. Studies have shown that surface roughness (fractal scale complexity) increases during storage, while gloss decreases1. Fractal analysis using cLSM techniques therefore provides an accurate method of observing and measuring roughness, for the description of surface changes of chocolate during storage.

Cosmetics

With an increasing incidence of eczema and allergy, it is progressively important that cosmetics are manufactured with rigorous quality control protocols in place. Cosmetics such as powder and foundation, which are commonly applied to the face, can be particularly problematic, causing irritation in such a highly sensitive area. As such, cLSM techniques can be used to examine the surface roughness of individual make-up particles. This inspection allows researchers to be sure that surfaces are extremely smooth, so as users do not feel the particles on their face. In addition, the surface of each make-up particle needs to be reflective and accurately colour-matched to varying skin tones. The accurate inspection of such products on the nano-scale therefore forms the crux of an entire industry.

Leisure

The sliding interface between a ski and the snow needs to incorporate a specific amount of friction to produce a smooth ride. In order to understand the wear mechanisms associated with friction and sliding on snow, the micro- and macro-scale geometry of the polyethylene ski bases needs to be analysed in detail2. However, there is no standard model to optimise the production of ski bases and thus the development of a surface with the desired texture can be problematic. In one study, five polyethylene ski bases were ground under different conditions and measured multiple times by a cLSM at three different sampling points3. The results of this investigation indicated that there are great similarities between the various worn textures on the finest scales, where the texture is formed by the interaction of an abrasive grit with the polyethylene. This also indicates that the surface textures at the finest scales tend to be most independent of the grinding conditions. The assessment of these circumstances and the wear on skis is subject to on-going research, in order to optimise the fine-scale composition of the base surface of skis to ensure a smooth ride every time, even after extended use.

Automotive

Rolling resistance occurs when a tyre rolls across a flat surface, at a steady velocity. This resistance is caused by a deformation of the tyre surface, the ground, or both. Additional contributing factors include wheel radius, surface adhesion and relative micro-sliding between the two surfaces 4. The process of tyre manufacture is therefore highly intricate, with various topological analyses performed to maximise the life-span of your tyres, while ensuring a safe, smooth drive. With implications in public safety, it is essential that tyres have enough grip with road surfaces to stop the car from sliding off the road, but at the same time allowing drivers to control speed and direction in an easy manner. cLSM techniques can therefore be used to ensure that tyre surface roughness is optimised to interact with the road surface in the most efficient manner.

Medical As the initial step in the diagnosis of various diseases, biopsies involve the removal of cells or tissues for further ex vivo examination. The extraction of these cells from a patient is a delicate procedure and can be performed using one of several different instruments, including: a fine needle and syringe, a hollow core needle, or a curved scalpel. As with any medical process, the risks associated with a biopsy need to remain minimal and as such, it is vital that there is little/no excess bleeding and no significant tissue trauma. In the development of biopsy needles, the surface needs to be extremely sharp, so as the biopsy can be removed using minimal force, thus decreasing the potential for undue trauma. Therefore, cLSM techniques have been used in the inspection of the scale and position of the radius of the cutting edge. This needs to be optimised in order to obtain the cleanest cut, with minimal off-target effects. With implications in patient safety, research into the efficiency of biopsy tools is pivotal to ensure that cells (for example, potential tumour cells), are extracted in a timely manner for in-depth analysis.

Forensics

Over the years, the manufacturing techniques used for the production of paper have developed significantly. Today, we are able to purchase paper with different textures and finishes, making it a complex process; one in which cLSM can be used to ensure that the required finish is obtained. In addition to its use in the manufacturing process, cLSM has also been used to date paper, for example in the detection of fraudulent paintings. Through the analysis of the paper used, investigators can estimate the date of production, thus providing an accurate method of identifying likely frauds.

History

When investigating evolutionary history, it is essential to delve into every aspect of life in order to glean as much information as possible. This enables us to gain and in-depth understanding of our ancestors and how we have evolved from them. Reconstructing the diets of extinct hominins (humans, chimpanzees, bonobo and their predecessors) has been carried out via the analysis of microscopic wear on the teeth.  Previously established methods of studying microwear have had issues with repeatability and high rates of human error. cLSM, together with scale-sensitive fractal analysis has been used in one study to characterise the complexity of the microwear in an accurate and repeatable manner5. Results for living primates have demonstrated this to be an effective method, which can distinguish between diet patterns via the characterisation of different fracture properties of teeth.

cLSM techniques are used by  a wide variety of industry sectors for the analysis of different surfaces. Microscopes such as the Olympus LEXT OLS4000 cLSM provide advanced functionality, enabling users to detect slopes up to 85°, for highly accurate surface profiling. It is essential that these measurement parameters are optimised for advanced accuracy, since there are numerous safety issues that could result in a vast array of application areas, from healthcare and cosmetics to food and automotive. With the general public being the consumer of the resultant products, optical metrology and surface topology are an essential component of our everyday lives.

References:

1. Briones V, Brown CA, Aguilera JB.  Scale-sensitive fractal analysis of the surface roughness of bloomed chocolate. J Am Oil Chem Soc 2006;83(3):193-199.
2. Mathia TG, Zahouani H, Midol A. Topography, wear, and sliding functions of skis. International Journal of Machine Tools and Manufacture 1992;32:263-266.
3. Jordan SE, Brown CA. Comparing texture characterisation parameters on their ability to differentiate ground polyethylene ski bases. Wear 2006;261:298-409.
4. Hibbeler, R.C. (2007). Engineering Mechanics: Statics & Dynamics (Eleventh ed.). Pearson, Prentice Hall. pp. 441–442.
5. Scott RS, Ungar PS, Bergstrom TS et al. Dental microwear texture analysis shows within-species variability in fossil hominins. Nature 2005;436:693-695.