Archimedes’ Principle and Surface Tension
Archimedes’ Principle
Archimedes lived from 287 -212 BCE. Among his most notable findings is the principle of buoyancy of a body, known today as Archimedes’ principle. This principle states that a body, whether partially or completely submerged within a fluid, experiences an upward force called buoyant force, or simply thrust, whose magnitude is equal to the weight of the fluid it displaces.
It is important not to confuse the weight of the fluid displaced by the weight of the submerged object.
Archimedes’ principle applies to the behavior of fluids in general. So a hot air balloon rises when its weight is less than the weight of the atmospheric air it displaces.
Why Does an Object Sink or Float?
The buoyancy of an object depends on the relationship between the density of the fluid in which it is submerged and the density of the object itself.
IN ORDER TO MORE DENSE FLUID:
In this case, the object will sink to the bottom of the liquid in which it is submerged because the object’s weight is greater than the weight of the displaced fluid, and therefore greater than the thrust.
The object has the same density as the fluid:
In this case, we cannot say that the object sinks or floats, since the weight of the object is equal to the weight of the displaced fluid and therefore equal to the thrust. However, the object may be at the surface of the fluid or at the bottom.
The object has lower density than the fluid:
In this case, the object remains partially submerged or floats. This is because the weight of the body, if fully immersed, is less than the weight of the fluid it displaces, so it rises to the surface.
Under these conditions, the floating object displaces a volume of water that is a fraction of the total volume of the object, which allows it to balance its weight and thrust.
In (a) The dynamometer measures the weight of the object. In (b) when the object is immersed in a fluid, the dynamometer measures less weight, which is known as apparent weight. In this case, the dynamometer reads less because the weight of the object is subtracted from the thrust force exerted by the water. This is a direct method to measure the thrust.
Surface Tension
Surface tension is the property that makes the surface of a liquid tend to contract, behaving like an elastic membrane. This explains the spherical shape of water droplets or oil droplets. Droplets are spherical because the surface tends to contract and make every drop into a shape of minimum area. This way, the area which is the geometric body occupies less space for a given volume.
Technically, stress is a force per unit length or, equivalently, the work per unit area needed to increase the surface of the liquid. In other words, the spherical shape of liquid droplets is the way that minimizes its energy. This force has a microscopic molecular level understanding. The molecules in a liquid are inside the attraction of other molecules. This force is of electromagnetic origin and is known as a cohesive force.
For each molecule on the surface of the liquid, the attractive forces act in all directions and as a result, there is no net force on each molecule, that is, the forces are in balance with each other. In contrast, molecules on the surface of the liquid in the lateral forces are balanced but the vertical forces are unbalanced because there are no other molecules of liquid over them. The molecular forces at the microscopic level are those that balance the small weight of the needle or clip.
To measure the surface tension, we use a procedure known as the Du Noüy method, invented by a French biochemist and mathematician. It consists of applying an upward force on a wire ring tied with a thread which rises gently from the surface of the liquid. Under these conditions, the surface tension prevents the immediate lifting of the ring. Surface tension is expressed in (N/m).
Capillary Action
When the end of a glass tube whose inner diameter is small is immersed in water, the water is able to climb the tube spontaneously. In a tube of 0.5 mm diameter, for example, the water rises about 5 cm inside the tube. This rise of water by a thin, hollow tube is called capillary action, since that type of item is called capillary (a word derived from Latin meaning hair).
Capillary action occurs because, in a way, the molecules of liquid are”sticky”
The attraction between molecules of the same substance is called cohesion. The attraction between molecules of different substances is known as adhesion.
When you enter the glass capillary into the liquid, the adhesion force causes the fluid to rise up the wall of the tube, while surface tension tends to contract the liquid film, rounding its contours within and outside the capillary. The surface of the liquid inside shrinks more and this increases the liquid through the tube until the weight is balanced by the force of adhesion. Thus, rising water for a thinner tube weighs less, so it reaches more height.
The relationship between the cohesive force of a liquid and the adhesive force presented to a solid, determines whether or not liquid spreads over the surface of the solid, i.e., if it gets wet or not.
If the cohesive force of a drop of liquid is less than the adhesion force between its molecules and the solid surface, then the drop is spread by the solid, wetting it.
On the contrary, if the cohesive force is greater, the solid is not wet.
A drop of liquid mercury, for example, spreads on the surface of a glass cleaner. The same goes for a drop of rain that falls on a freshly waxed car, the drop does not wet the car, but rather due to the zero slip grip.
The adhesion capacity of a drop with a solid surface versus the ability of cohesion of the liquid molecules can be quantified through an easily measurable angle called contact angle.
If the contact angle is 0
According to the above, a liquid cannot wet the inside of a capillary. In the event that the liquid does wet the walls of the tube, because the bond strength is greater than that of cohesion, there is a concave upward or concave meniscus fluid.
If the liquid does not wet the walls of the tube, it produces a concave or convex meniscus downward. In this case, there is no capillarity, but on the contrary, the liquid goes down the tube.
Capillarity Occurs Everywhere
Capillarity is a fundamental phenomenon in many natural and artificial situations.
Before reaching the plant roots, thanks to the capillary water that falls on land is distributed by the micro air gaps between the soil particles. After the transport of water and other substances from roots to leaves in plants is a problem in plant physiology in which the capillarity plays a crucial role.
The water enters the roots through root hairs, enters a system of interconnected cells that form the fabric of the plant and extending from the same roots to the leaves through the trunk or stem. This woody tissue called xylem, consists of several cell types. The rise of raw sap is favored by the small size of woody vessels that attach to water molecules, as the ascent is more effective the smaller the vessel diameter, ie, by capillary action.
However, capillarity is not sufficient to raise the water to all parts of the plant. Several additional processes are required for this to happen, among which the most important is the evaporation of water molecules through the leaves. Since water molecules tend to bind to each other through sheer force of cohesion, when a molecule evaporates through the pores of a leaf, has a small boost to the adjacent molecules, which reduces pressure on the woody cells and draws water from adjacent cells. Call this effect extends all the way to the roots and joined the capillary effect.
In the circulatory system of our bodies also occurs capillary phenomenon. Some ten billion capillaries are interwoven into all the tissues of the body, providing toas blood cells. They are the smallest blood vessels, microscopic in size and contain less than 5% of the total volume of circulating blood.
Capillary technological objects are in many cases: sponges, paper towels, cloth, alcohol burners, lungs ink, pens, etc. Even the walls of a building is damp and deteriorate because the water goes up inside due to the same phenomenon.