A meniscus is a curve in the upper surface of a liquid close to the surface of a container or another object, caused by surface tension. It can be concave or convex (the vertex is at the top or bottom).
The shape of a meniscus depends on the substance that it is made from. For example, a water meniscus is curved when the force of attraction between molecules of water and the glass is greater than the cohesion between the water molecules themselves.
Surface Tension
The shape of the upper surface of liquids in a tube or other object, called meniscus chemistry, is a measure of the balance between adhesive and cohesive forces. It is also a measure of the viscosity of the liquid.
In general, polar liquids have higher surface tension than nonpolar liquids. This is because they have stronger intermolecular interactions.
Water is a good example of a liquid with strong intermolecular interactions. If a water droplet is placed on a flat surface, its molecules will be attracted to each other. This causes the droplet to adopt a spherical shape.
It is interesting to note that water molecules have different energy states in the interior and exterior of a water droplet. The interior molecules are attracted to all the other molecules around them, while the exterior molecules are only attracted to the other molecules on the surface of the droplet. This difference in the energy state of the molecules causes them to adopt a different shape and creates what is called surface tension.
This is because the molecules on the outside have a larger amount of energy than those on the inside. Because of this, the exterior molecules try to maintain a minimum surface area.
Likewise, the water molecules on the surface of the droplet try to keep their distance as low as possible. This means that their attraction to each other will be greater than their attraction to the other molecules on the surface of the water.
In addition to this, the molecules on the bottom of the droplet have a higher energy state than those on the top. This creates an inward force on the molecules on the bottom of the droplet. This force causes the molecules to contract and resist being stretched or broken.
These forces are important in explaining why a glass-water meniscus is concave and a mercury-glass meniscus is convex. The mercury drops have more adhesive forces on the surface than the water drops do.
Cohesion
Cohesion is the attraction between like molecules on a liquid’s surface. In most liquids, this is a strong force. It allows light objects to float on water without sinking. It also keeps droplets of water from falling off surfaces.
When water or other cohesive liquids are placed in a glass tube, they tend to form a meniscus that is slightly curved upwards. It may be concave or convex, depending on the balance of adhesion and cohesion forces at the interface between the liquid and the glass.
If the liquid molecules are more attracted to the glass wall (adhesion) than to their own kind (cohesion), they pull down on the liquid’s surface and raise the glass edge. This results in a concave meniscus.
Another example of the cohesion/adhesion ratio affecting meniscus shape is mercury in a glass capillary tube. If a lot of mercury is placed inside the tube, it forms a convex meniscus.
This type of curve is more pronounced in thin tubes than in thick ones. However, the underlying mechanism is the same.
In thin tubes, the liquid’s molecules are more attracted to the glass wall than to their own kind, so they “pull down” on the liquid’s surface and raise the edge of the glass. The resulting surface tension, in turn, results in the concave meniscus of water.
The meniscus of water in a plastic tube, for example, is nearly flat. This is because the adhesive force between the water molecules and the surface of the plastic is stronger than the cohesion force between the molecules themselves.
Similarly, the mercury atoms in a silica glass capillary tube are more attracted to the silica glass wall than to the body of liquid mercury. The resulting surface tension, in turn, is the reason why the meniscus of mercury in a glass capillary tube is more curved than in a plastic tube.
The same is true for a variety of other liquids. For example, when a glass capillary tube is filled with alcohol, the dribbles of liquid appear to “float” above the meniscus of the liquid and form “tears.” This phenomenon is known as the “solutal Marangoni effect.” It is caused by surface tension, cohesion, and adhesion forces.
Adhesion
The interplay between adhesion and cohesion causes a meniscus to form in a liquid. It can be concave or convex, depending on whether the liquid molecules are more attracted to the outside material than they are to themselves (adhesion) or vice versa (cohesion).
The most familiar example of a meniscus is that of water in a glass tube. This is because water has a high cohesion force, which means that its atoms stick together well. But its adhesive force is much stronger, so it tries to stick to the glass wall. The result is a concave meniscus.
Another example of a meniscus is that of mercury in a glass tube. This happens because mercury has a higher adhesive force than water does, so the meniscus is curved upwards.
A similar effect is seen in some types of plastic tubes; these are often made from materials that water does not stick to. They can have either a flat or a concave meniscus, depending on how much the molecules of the liquid are more attracted to the outside material than to themselves.
For example, glass is polar, meaning it dissolves in water like glucose (a polar molecule), but plastic is non-polar. Because of this, glass does not have an attractive attraction to the molecules of water, so it forms a concave meniscus.
This can happen for all kinds of liquids, but it is especially true for aqueous solutions because the cohesion forces in these are more strong than the adhesive forces that water has. This is why, if you take a glass pipette filled with liquid and put it into a bottle of water, the water will flow down the bottle and not up into the tube.
But if you put the same glass tube into a bottle of mercury, the molecules of the liquid will stick to each other better than they will to the wall of the container. This results in a convex meniscus.
For meniscus repair, tissue adhesives should be designed to resist physiological stresses, so that they are able to stabilize the meniscus and prevent gap formation. They should also be able to hold a tear measuring 1 cm2 under a stress of 50-100 kPa, which would be the minimum necessary for a successful meniscus repair in the non-weight-bearing recovery period. In addition, tissue adhesives should be able to incorporate cells or growth factors, which may improve the healing process.
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Forces
The meniscus has a variety of forces acting on it during knee movement. These forces are responsible for joint load transmission, joint stability, and proprioception. They also act to maintain a homeostatic environment for the knee joint.
The medial and lateral menisci are C-shaped wedge fibrocartilagenous structures located between the condyle of the femur and the tibia. In addition, the menisci are attached to the tibia and femur via a variety of ligaments (see diagram below).
However, little is known about the loads that act on these attachment structures under physiological joint loads and movements. Therefore, it is important to investigate the effect of these loads on the attachment structures in a variety of settings.
This is accomplished by analyzing the force profile of the medial and lateral menisci under various loads in different motions. This is an essential step in designing and developing meniscal substitutes that can replace the damaged menisci.
In this study, a knee model was used to measure the tensile forces that were produced by combined flexion and rotation of the joint during weight-bearing and non-weight bearing conditions. The tensile forces generated were influenced by the flexion angle and direction of rotation.
These tensile forces ranged from 0.5 to 4.1 N and were significantly lower than the minimum strength assumed for common meniscal repair techniques. These results indicate that flexion and rotation are not significant forces for the meniscal suture.
The elasticity of a fibrocartilaginous tissue like the meniscus is controlled by its permeability. As a result, if permeability increases, the tissue loses its ability to retain its shape. The meniscal tissue is one-eighth as permeable as articular cartilage. This permeability allows the meniscus to withstand compressive and tensile stresses that would otherwise deform or fail.
