Adhesives play a pretty important role in our everyday lives; even if we aren’t always aware of them, they’re present in the world all around us. Whether it’s the wood glue holding our tables and chairs together, or the duct tape that we haphazardly throw on something in a desperate attempt for a quick fix, adhesives have contributed a lot to modern society. But how exactly do these miracle sticky substances work? Let’s take a look at the science behind adhesion and different types of adhesives.

Let’s begin with the history of adhesives. The first uses of adhesive substances may date all the way back to Ancient Egypt, where they used natural glue made from animal collagen. Other ancient societies used things like tar and tree resin. Other natural substances, like beeswax, have been used as adhesives for many years as well.
But what exactly are these adhesive substances, and what makes them so sticky? An adhesive can broadly be defined as “a substance capable of holding materials together by surface attachment.” They are usually a liquid, solid, or paste and attach to (or stick) materials in three ways:

• mechanically
• chemically and/or
• electrostatically

Their degree of stickiness can change depending on the combination of these interactions.

The most recognizable type of adhesive is tape, which is a pressure-sensitive adhesive (PSA), a type of polymer that has a very high viscosity and some elastic characteristics. This means that the PSA has a high resistance to flowing, like pancake syrup, and can resist outside forces, retaining its shape if it is moved like a rubber band. Despite being a solid, PSAs have some liquid characteristics, so they will “wet” a surface when applied to it, but they will also resist separation when stressed because of their elasticity. The resulting stickiness is enhanced and removable due to this “viscoelastic” characteristic.
There are two major interactions that contribute to PSA’s stickiness: mechanical (the wetting process) and electrostatic (Van der Waals forces). Wetting means a solid adhesive can spread across and be absorbed into the material to which it is being applied. Think of water beading up (non wet) on the surface of a freshly waxed car versus spreading out on a wooden table (wet). Wetting typically occurs because the surface tension of a substance is lower than the surface on which it is. Because solid adhesives have a low surface energy, they are able be absorbed into the material, allowing for the surface molecules of the adhesive to flow easily into the pores of the material across a wide area.
As this is happening, the tape is also electrostatically binding to the material with the creation of Van der Waal’s forces- weak attractions between usually neutral molecules whose charges are not evenly distributed, creating a dipole moment. These charges, or polarities, enable the molecules to form bonds with other polar molecules. The molecules of PSAs exhibit these dipole moments and consequently create corresponding dipole moments in the molecules of the surface to which they are binding. Therefore, all it takes is a little pressure in order to force these physical bonds between the material and the adhesive. This also means, however, that no chemical reaction takes place and it is strictly a physical phenomenon.
Glue, on the other hand, is a chemical and sometimes mechanical interaction which has stronger bonds. When you use glue to stick two materials together, the chemistry of the glue can change so that the glue molecules permanently bind to themselves (hardening). This means that these bonds are irreversible; if an object breaks after glue has been applied to it, the same glue cannot be used to put it back together. The bonds formed by glue are stronger and more permanent than the bonds formed by tape and other similar adhesives.
Although we have come a long way from using beeswax and animal collagen as adhesives, we are still searching for more and better adhesive materials. Niels Holten-Andersen, John Chipman Assistant Professor of Materials Science and Engineering at MIT, is currently researching the adhesive powers of aquatic animals. “Mussels and shellfish produce fibers that allow them to stick to ships and rocks and anything they want to grow on,” Holten-Andersen says. Understanding how these organisms create adhesive fibers could prove helpful in creating synthetic glues that will work underwater, and could have numerous medical applications, such as for organ transplants. Clearly, adhesives are a force to be reckoned with!

Sources:
http://engineering.mit.edu/ask/what-are-basic-forces-behind-tape-and-glue
http://science.howstuffworks.com/innovation/everyday-innovations/adhesive-tape1.htm
Brinson, H. F. Adhesives and Sealants. S.l.: ASM International, 1990. Print.

Did you miss out on our January webinar – Visualizing Hydration and Dehydration of Pharmaceuticals, Foodstuffs, and Other Materials Using Wet Scanning Electron Microscopy? Don’t worry about it, because you can learn about everything that you missed right here or even watch the recorded webinar on demand here: http://analyticalanswersinc.com/about-us/webinars/

Edward Norton, Technical Director at Analytical Answers, presented this session. It focused on the abilities of wet scanning electron microscopy (SEM) in solving challenging problems across many different industries from pharmaceuticals to the food industry.

To begin the webinar, the basics of scanning electron microscopy were reviewed. So, how does it work? An electron source running in a high vacuum produces an electron beam. The beam passes down the column towards the sample. On its way down, coils and lenses scan, deflect, and focus the beam onto the sample. The beam then interacts with the sample and produces secondary and backscattered electrons and x-rays. Scanning electron microscopes are a very useful tool in giving us high resolution, high magnification images.

A wet SEM, or an environmental SEM, is a SEM in which you introduce gas and water vapor and control the humidity. Technologically, a wet SEM differs from a normal SEM by:

• A specialized pumping system, which allows the chamber to be at higher pressures than the electron source
• A cold stage, which allows for wet conditions
• A water source, and
• Specialized detectors, which work at higher pressures than in normal SEM conditions and allow for the greatest surface resolution.

Samples in a wet SEM can start wet or can start dry and become wet. In the first example you are viewing the sample in its natural state. In the latter example, you can observe the sample changing phases from dry to wet. To best capture, visualize, and understand the changes happening to the sample when going from wet to dry, it is best to do cycles. This is when the sample is reproducibly exposed multiple times in succession, which allows us to look at multiple exposure times during specific humidity conditions.

Wet scanning electron microscopy is used for three main reasons:
• The first is charge neutralization. In normal SEM samples must be conductive or made conductive for the technique to work. In wet SEM the gas and water molecules in the chamber perform this function for us.
• The second reason is to prevent dehydration. This is accomplished by examining wet samples and reducing and/or controlling the rate at which they dry out.
• The last reason is to observe phase and material changes. After a dry sample is exposed to humidity, the changes of the sample’s texture and structure are recorded. This can also be used to observe dry samples at higher and higher temperatures to look at deformation as a function of temperature over time.

Although wet SEM is a very useful tool, it does have some limitations. Pressure is significantly below atmospheric pressure, so if the material of the sample is very porous, you may get some gassing/bubbling. Low temperatures are required to achieve condensed (wet) conditions, so reaction kinetics are reduced. Therefore, you must keep the temperature at a constant for a longer period of time, if you are trying to gauge reaction kinetics. Another limitation is the video speed; the fastest video that can be captured is about 1 frame per second due to the gas scattering time. The last limitation is that small sample sizes are required, due to the small stage area.

Analyses are performed by either: 1. Controlling the pressure and altering the temperature or by 2. Controlling the temperature and altering the pressure. Constant temperature is better when monitoring reaction kinetics, but constant pressure is better for video analyses.

An example of a case in which wet SEM is very useful is with a pharmaceutical product – a capsule shell. A capsule shell is the outside casing of a medication. How fast the capsule dissolves needs to be regulated by producers in order to control the rate at which the drug inside the capsule is released and then absorbed by the body. By using wet SEM, different formulations of capsules can be compared to find the perfect dissolve rate.

Another example of a case in which wet SEM is useful is with food products. In wet SEM you can visualize the morphological and textural changes that occur to a food sample at different humidity levels. This can help food producers to be more knowledgeable about the necessary conditions that food products need to be stored in.

Wet SEM has a wide range of applications in pharmaceuticals, food science, and other industries where analyzing wet samples is critical to quality control. It allows for testing of products and substances in ways that simulate real-world conditions, providing insights into performance.

Have you ever thought about how we detect smells, or how we determine if it is a good smell or a bad smell? This blog will give you some insight on just that, as well as a smell that many of us enjoy – the smell of rain.

To smell an odor air is inhaled through the nose. The inhaled air carries odor molecules. Once in the nose, the molecules then dissolve into a mucous membrane called the olfactory epithelium. In the olfactory epithelium the molecules are able to spread out and bind to receptors on the tips of dendrites of olfactory neurons. (All the olfactory neurons bundle together to form the olfactory nerve.) Once molecules bind, the olfactory neurons start firing signals to the olfactory bulb in the brain.

The olfactory area in the brain is closely connected to the amygdala and the hippocampus. The amygdala is involved in emotion and the hippocampus is involved in memory. Therefore, the sense of smell is linked very closely with emotions and memories. A smell can remind us of a certain memory or feeling and thus force us to connect the smell to something we perceive as positive or negative/pleasant or unpleasant. Moreover, some smells which all humans regard as bad may be due in part to evolution. For example, the smell of rotten food and the smell of mildew may be unpleasant to us because they are warnings of danger.

Since the science of smell has just recently spiked scientists’ interest, the relationship between the molecular composition of an odor molecule and how we perceive the smell of the molecule is still largely unknown. One hypothesis states that different odor molecules fit differently into receptors, which in turn makes the pattern of firing signals to the brain different. In this hypothesis, the different patterns determine how the scent is perceived.

Now, let’s think about a smell that many of us perceive as pleasant – the fresh, earthy smell of rain, which is actually referred to as ‘petrichor’. A common question might be – why does rain have a smell, if it’s only water molecules? The answer is that the odor we are perceiving is not actually the rain itself, but what is released when a rain drop hits the surface of earth.

The scientists who named and first described petrichor were Isabel Bear and Richard Thomas. They performed a series of experiments, and published their results in ‘Nature of Argillaceous Odour’ in 1964. By steaming warm, dry rocks, they concluded that smell of rain actually came from an oil that was trapped in the rocks and released by the moisture.

More recently, a team of scientists led by Cullen Buie at the Massachusetts Institute of Technology decided to look more closely at the phenomenon of petrichor. They dug deeper than Bear and Thomas could have using modern technology, more specifically a system of high-speed cameras. Buie and the team filmed water droplets hit 38 different surfaces, including engineered materials and soil samples.

The resulting videos, when viewed in super slow motion, showed what the team called “the petrichor process.” When water droplets hit the surface of a porous material, they trap the air present in the material in tiny pockets. The air pockets, being less dense than the water, speed upward, break the droplets surface, and release particles into the air, including aerosols. The aerosols, the team thinks, are responsible for the “smell of rain.” By filming drops at different speeds on different surfaces, the team was able to conclude that the amount of aerosol particles released coordinated with the velocity of the droplets and the permeability of the material on which the droplets hit.

Many scientists were quick to point out that this process was a possible mechanism of spreading soil-based contaminants. In the future, scientists will try to decipher if viruses and bacteria are actually able to spread this way, and if so, how far.

Sources:
http://www.fifthsense.org.uk/what-is-smell/

http://www.dana.org/Cerebrum/2001/Ah,_Sweet_Skunk!_Why_We_Like_or_Dislike_What_We_Smell/

http://news.mit.edu/2015/rainfall-can-release-aerosols-0114

http://www.nature.com/articles/ncomms7083

http://earthsky.org/earth/whats-that-smell-in-the-air-when-its-about-to-rain

http://www.livescience.com/49520-smell-of-rain-aerosols.html

http://www.livescience.com/37648-good-smells-rain-petrichor.html