The ability of getting wet

Rodolfo Marin Rivera

1Have you ever wondered why when the rain stops, water drops remaining on the surface of the leaf do not flow or roll? Well that’s because of the surface tension of the water and the hydrophobicity of the leaf… but wait a minute! What’s surface tension and what’s hydrophobicity?

Both terms can be explained by considering Young’s principle about thermodynamics of wetting. Young described the ability of a drop to spread or not on a surface as the wettability of the surface, and he correlated this phenomenon by measuring the contact angle formed between the liquid, the solid and the gas around these two phases, as it is presented in this diagram:

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If a drop of a liquid is put it on a surface (as the leaf shown in the picture) there are two possibilities:

  1. The liquid spreads on the surface completely, so that the red arrow squash the angle θ formed between the green and the red line (see the drop of liquid at the left), which means the contact angle θ becomes equal to zero, or
  2. A “contact angle” is formed between the liquid, the solid and the gas phase, so that these three phases are in equilibrium and the contact angle θ tends to 180°, as  described by the drop of liquid in the right.

In other words, one can assume that the drop will wet the surface as long as the contact angle θ tends to zero, as the handsome drop of liquid in the left. Therefore, if the drop of water remains on the surface without spreading, then it is said that the surface is hydrophobic (no wet), but if the drop spread into the surface then the surface is called hydrophilic. It may occur that a liquid can be partially adsorbed on a specific surface because the wettability of a solid surface also depends on the surface tension between the liquid, solid and gas phases.
On the diagram on the left, the surface (or interface) tension is defined by the intermolecular forces that attract the liquid particles together. Along the surface, the 3particles are pulled toward the rest of the liquid. Surface tension (denoted with red arrows and described by the greek letter gamma, γ) is defined as the ratio of the surface force F to the length d along which the force acts:

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The term “surface tension” is used when a liquid in contact with the gas phase acts like a thin-elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air around the leaf, or the foam formed when you’re taking a very nice bath), but If the surface is between two liquids (such as water and oil), then the term “interface tension” is used.

Young correlated the contact angle with the surface tension between the different phases with this very simple equation

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where γlg represents the surface tension between the liquid and the gas phase, γsl represents the surface tension between the solid and the liquid phase and, γsg describes the surface tension between the solid and the gas phase. The location of these surface/interface tension are shown in the picture above with the two drop of liquids.

Therefore, wettability of a surface is controlled by the first term of the equation, which is known as the “adhesion tension”. If θ becomes equal to 0°, and, for instance, the left term in Young’s equation becomes equal to γlg, which means that the liquid wets the solid surface perfectly. But, if the adhesion tension becomes smaller than γlg, it means that cos θ < 1 , and therefore θ > 0°, which means partial wetting of the solid.

For more info about wettability you can check the following links:
https://www.sciencedirect.com/science/article/abs/pii/S0301751617300510

https://www.spec2000.net/09-wettability.htm

http://web.mit.edu/nnf/education/wettability/wetting.html

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Soft robots inspired by nature

images (1)Nature is always of great inspiration for the development of novel materials with biological applications. This is what the research team of Prof Ali Khademhosseini did by creating hydrogel based soft robots that allowed the growth of cardiomyocytes and showed self-actuating motions aligned with the contractile force of the cells.

These soft robots were fabricated by mimicking the biomechanical model of a batoid fish. Find out more about the different components of the robots and how they work in this article!

Thin films with antireflective/antibacterial properties

downloadHave you ever thought that touchscreens can host bacteria? Think about it and you will realise why this happens….However chinese researchers have created new silica thin films containing silver nanoparticles, which not only have antibacterial properties but are also antireflective.

These new materials are very promising for technological applications…Curious to know more? Read the full article here!

What does make a radioactive element radioactive?

Rodolfo Marin Rivera

1The periodic table as we know contains 118 elements,

which are organised according to their atomic number, electron configuration, and chemical properties thanks to discovery done be Dmitri Mendeleev (1869). However, all of them may have some level of radioactivity.

As we know, atoms are made up of small, positively charged nucleus surrounded by a relatively large space (orbitals) occupied by tiny, fast-moving, negatively charged electrons, which are 10,000 to 100,000 times smaller than the atoms. Despite its relatively small size, the nucleus contains over 99.9% of the mass of the atom. A very close approximation about what is going on during the interaction between the electrons and nucleus was given by Bohr (1913) who depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus with attraction provided by electrostatic forces.

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The nucleus is made of positively charged protons and neutral neutrons. The number of protons is equal to the atom atomic number and determines, for instance, what element it is. For example, an atom with 8 protons in its nucleus is an oxygen atom, the eight-element listed in the periodic table. The number of neutrons will simple affect the mass of the atom and determine which isotope of oxygen is. Therefore, every chemical element has one or more isotopes that, for a given element, the number of protons and neutrons may change or not. Isotopes are better known as nuclide. Nuclide refers to any isotope of any element, or in other words, a nucleus with any number of protons and neutrons. Therefore, the radioactivity of a certain element is given by the transformation of a certain nuclide, or by the intensity at which the nuclide may decay. This means that a radioactive nuclide is one that spontaneously undergoes nuclear decay.

3During nuclear reactions (as the big bang), the interaction between a particle (like a neutron) or a photon with a nucleus leads to the formation of another nuclide. This transformation is known as nuclear decay. A nuclear decay stars with an unstable nuclide (parent isotope) that typically spits out a particle and/or a photon while turning itself into something more stable. Therefore, energy is released as part of all nuclear decay. There are three different kinds of decay: alpha (i.e. ejection of neutrons and protons at once, formation of the helium atom), beta (ejection of electrons) and gamma (no particles are emitted or absorbed, and both the numbers of protons and neutrons remains the same). In this last type of decay, at the beginning, neutrons and protons are arranged poorly, in terms of energy. That is to say one or more neutrons/protons occupy a higher energy state than they need to, and that the nucleus is said to be in an excited state. Rearrangement allows the neutrons/protons that are in unusually high-energy states to drop down into lower ones. The lowest energy configuration would be called the ground state. Rearrangement from a higher energy state to a lower one requires the release of energy, which can be done in the form of a high-energy photon, or gamma ray.

Nowadays, about 20 radioactive nuclides are recognised, which will take a very long time to decay. One of the most notable is 40K. Since all isotopes of an element have the same chemical behaviour, and because we humans need potassium in our bodies, we all have some 40K in our bodies. Anything that contains potassium (bananas, Gatorade®, your friends) is naturally radioactive, i.e. of natural radioactive origin (also known as Naturally-Occurring Radioactive Materials, NORM).

All but three of these 20 long-lived radioactive nuclides decay to stable nuclides. The three that don’t are 232Th, 235U and 238U. They all eventually decay to stable isotopes of lead, but along the way, they produce a number of other naturally occurring radioactive nuclides (e.g., 222Rn).

4You must know that from all the ionizing radiation that exist around us, not all of them is related to our activities as human being, because radionuclides exist everywhere in nature (in soil and rocks, air and water, in plants and in our bodies). Some of the isotopes have been there since the creation of Earth, others are being constantly produced or brought in from outer space as cosmogenic radiation.

Like this topic? You can  find more information in the following links:

 

 

Cell-sized robots that can change their shape

imagesResearchers at Cornell University built origami machines that can modify their shape in response to different stimuli and carry loads such as embedded electronics. These small robots are extremely small and are produced by graphene actuators that can fold 2D patterns into targeted 3D structures.

This research has no application for the moment, however it opens new paths to robotics for cells and biological systems.

Getting interested? Read more:

 

 

3D-Printing human tissues using cellulose: will science fiction become reality?

Carmen Piras

imagesMost of you have probably heard about the Frankenstein’s monster, a novel written by the English author Mary Shelley. For those who don’t know it, it is the story of Victor Frankenstein, a scientist who invented a secret technique to impart life to unanimated matter. This technique allowed him to create a monstrous creature with human emotions and sensations.

This is of course only science fiction. However, nowadays, the on-demand growth of human tissues and organs using artificial instruments is becoming reality. One emerging manufacturing technique with promise in tissue engineering and regenerative medicine is 3D printing, which is based on the 3D deposition of material in specific shapes. This technology can be applied in biology (3D bioprinting) to produce 3D scaffolds of our tissues. The cells are directly embedded inside the scaffolds and different cell types can be distributed in different locations. The material loaded with the cells is then 3D printed into the desired shape. This method allows the fabrication of bone and cartilage tissues, skin and cardiac constructs and the regeneration of hepatic tissue.

One of the most important factors in 3D bioprinting is the choice of the material used as ink (or, more specifically, bioink), which should be biocompatible and allow cell survival during the printing process. Beside this, the ink should have an optimal viscosity to allow printing and to avoid the 3D printed shape to collapse. Water based gels (also called hydrogels) are ideal inks for 3D bioprinting. These materials are mainly composed of water (> 99 %) and therefore can closely mimic the natural environment of cells.

Hydrogels can be obtained from a wide range of molecules including natural derivatives such as gelatine, alginate, collagen, hyaluronic acid and cellulose. Being an abundant, renewable, low-cost resource, cellulose represents an ideal candidate for the production of hydrogels for 3D bioprinting. This molecule is formed of long glucose chains and it is obtained by extraction from plants or can be produced by bacteria. Mechanical and chemical treatments of raw materials allow the extraction of cellulose in the form of nanofibers or nanocrystals. The suffix nano- refers to the fact that the extracts have one dimension (length or width) in the nanometre range (1.000.000 smaller than 1 mm).

UnknownA variety of bioinks based on cellulose nanofibers and nanocrystals has been created by a number of research groups. These could be applied for the regeneration of human tissues (such as cartilage) or to obtain 3D printed drug delivery systems and wound dressings. Although very promising, this research field is still new and growing. We hope that in the future cellulose will be further exploited to develop new bioinks. Will this be the route towards a new Frankenstein’s monster? Stay tuned!

Want to know more about this? Read this article: http://pubs.rsc.org/en/content/articlepdf/2017/bm/c7bm00510e

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