Why is a drop of water shaped like a ball? Why is the drop round?

"article" A drop of water - as it is". Where we'll talk about what a drop of water is, how it differs from a non-drop, and other interesting things.

A drop of water - as it is - is one of the ways to take a closer look at the world around us. Look at it with different eyes, from a different angle - not the usual one, but a different one. In our case, it’s a little more scientific.

So, in most cases, a drop of water is perceived as a ball of water, which is annoying when dripping from a tap, and makes you happy when it’s raining outside the window.

But this is only at first glance. So, according to the dictionary:

A drop is a small amount of liquid that takes on a rounded shape due to the adhesion of its particles. The weight of a drop depends on temperature, on the substance of the body from which the drop is separated, on the size of this body and on the surface tension of the liquid.

A drop is a small volume of liquid limited by a surface of rotation or close to it. The shape of the drop is determined by the action of surface tension forces and external forces.

Surface tension- this is the force with which molecules of a substance are attracted deep into the material. There are, of course, more abstruse explanations (surface tension is the work of reversible isothermal formation per unit area of ​​this surface, BES material). But in fact, everything is quite simple. In the case of water, the surface tension of water is nothing more than water molecules attracting each other. Like iron dust around a magnet.

So, we have two forces - water molecules attract each other. Accordingly, when they attract each other under certain conditions, drops are formed.

Conditions for droplet formation:

• when liquid drains from the edge of the surface or from small holes (the same drop falling from the tap).
• when steam condenses:
  • a) on a hard non-wettable surface;
  • b) at condensation centers. (example: when something brought in from the cold fogs up).
• when spraying liquid (by the way, spraying liquid is used in fire fighting).
• emulsification (mixing one liquid in another, insoluble in it; example - emulsification occurs when oil and water are mixed).
• Dew is formed by the condensation of water vapor on surfaces, fog and clouds are formed by the condensation of water vapor on dust particles in the air.

In each case, circumstances form very small quantities from water. Well, then our surface tension studied above comes into force.

So, drop shape is determined by the action of surface tension (we have already determined what it is) and external forces (primarily gravity). Microscopic drops, for which gravity does not play a determining role, have the shape of a ball - a body with a minimum given volume surface (since water molecules are evenly attracted to each other). Large drops under terrestrial conditions have spherical shape only when the densities of the liquid drop and its surrounding environment are equal.

Falling raindrops, under the influence of gravity, the pressure of the oncoming air flow and surface tension, take on an elongated shape. On non-wetted surfaces, droplets take the shape of a flattened ball. By the way, raindrops cannot be larger than 5 mm, since large drops break up in the air.

The shape of the drop is aerodynamically optimal, since it has a surface that least interferes with air resistance during flight.

So, a drop of water, as it is, is a coincidence.

Some of which are responsible for grinding water into small portions, while others are responsible for attracting water molecules to each other.

Based on materials from http://voda.blox.ua/2009/05/Chto-takoe-KAPLYa-VODY.html

We are accustomed to the idea that a drop has the shape of a ball. In fact, it is almost never a ball, although this shape provides the least volume.

A drop resting on a horizontal surface is flattened. Complex shape has a drop falling in the air. And only a drop in a state of weightlessness takes on a spherical shape.

In big Soviet encyclopedia Instant photographs of falling raindrops are shown. In particular, a droplet with a diameter of 6 mm has a shape close to the shape of a mushroom cap; Drops of smaller diameter have a shape close to a ball.

The formation of a drop can be described in three ways: characteristic conditions. State A corresponds to the beginning of the formation of a drop: the surface of the liquid at the end of the tube is horizontal, the radius of its curvature is very large, the surface tension forces are directed perpendicular to the wall of the tube and do not prevent the liquid from flowing out. Through a short time the drop goes into state B, which is characterized by the greatest Laplacian force, which slows down the rate of droplet formation and, consequently, the rate of outflow. In this state, the radius of curvature of the surface is r. Then the volume of the drop increases, it passes into state B, which characterizes the main stage of drop formation: the Laplace force is large, but less than in state B, and further decreases with increasing radius of the drop; The time of accumulation of the mass necessary for separation is long compared to the time of transition from state A to state B, the rate of leakage further decreases.

Drop radius

Falling raindrop, due to relativity mechanical movement, can, to a first approximation, be replaced by soaring drops in an ascending air flow.

We repeated the experiment described in the journal. The drops were placed in an air stream using medical syringe. To do this, the end of the needle was placed in a stream of air, and by slowly squeezing water out of the syringe, drops of various volumes were obtained. Drops, due to wetting, can remain on the needle for some time. At this moment, you can clearly observe the shape of the drops. After some time, the drop falls from the tip of the needle and hangs in the air for a few seconds. This time is sufficient to examine the shapes of drops of various sizes or photograph them.

In the course of the study, it turned out that drops of small diameter actually have a shape close to a ball, and drops of larger diameter have a shape reminiscent of a mushroom cap.

Observation of the decay of a drop into a ring and the interaction of rings

We decided to observe the disintegration of a drop into a ring to verify the validity of the data presented by the authors on the behavior of an ink drop on the surface and inside water. During the experiment, we recorded that a denser liquid tends downward according to the laws that are described by the Rayleigh-Taylor instability, with the formation of vortices.

To do this, we used a transparent glass vessel that was filled with water. Selected capillaries various diameters and thus obtained drops of different radii.

The behavior of an ink drop depends on several parameters: if the liquid has high density, for example, solution table salt, or a drop falls from high altitude and hits the surface of the liquid at high speed, it breaks into pieces and does not penetrate deeply into the liquid. But if the density of the liquid is slightly less than that of ink, and the drop falls from a height of several centimeters, then interesting transformations occur with it.

If you carefully bring a drop of ink to the surface itself and touch it, the drop will be instantly drawn into the water and begin to move down at high speed. The drop acquires this speed under the influence of the mutual attraction of liquid molecules. The forces that arise in this case are called surface tension forces because they always tend to reduce the free surface of the liquid, drawing it inward and leveling out any unevenness on it.

At first, the ink drop plunges into the water at high speed, but then its movement slows down. The reason for this movement is the Archimedean force, which almost balances the force of gravity, and the frictional force between the drop and the stationary water. Since the frictional force acts only on the outer surface of the drop, after traveling a few centimeters, the drop turns into a rotating ring.

The mechanism of formation of a vortex ring is quite simple: side surface the drops are slowed down by the still water and begin to lag behind the inside. The place of the failed middle is taken by clean water.

The ring does not remain perfectly round for long: its rotation slows down, and bulges and depressions appear on it. This phenomenon is called Rayleigh-Taylor instability, which consists in the fact that a layer of heavy liquid lying on a layer of lighter liquid may be in equilibrium, but this equilibrium will be unstable. As soon as the interface between liquids bends a little, the heavy liquid will rush into the depressions, and the light liquid will begin to float, increasing the swelling. This is completely natural: liquids tend to occupy a position of stable equilibrium, when the light one is at the top and the heavy one is at the bottom.

The movement of a jet in a stationary liquid is in many ways reminiscent of the movement of an individual drop: under the action of viscous forces, a vortex ring is again formed at the end of the jet, which in a few seconds, under the influence of Rayleigh-Taylor instability, will itself generate 2-3 jets. This “budding” process is repeated several times until the ink reaches the bottom of the jar, leaving a trail behind it.

When studying the interaction of vortex rings, the moment they are at the same height, they begin to interact with each other. There are three possible cases.

The first case is when the second ring overtakes the first without touching it. The following happens. Firstly, the flow of water from both rings seems to push the rings away from each other. Secondly, the flow of ink from the first ring to the second is detected: the water flows of the second ring are more intense, and they carry the ink along with them. Sometimes some of this ink passes through the second ring, resulting in the formation of a new small ring. Then the rings begin to divide; then we were unable to notice anything interesting.

The second case is when the second ring touches the first one when overtaking. As a result, more intense flows of the second ring destroy the first. As a rule, new small vortices are formed from the ink clot remaining from the first ring.

The third case is when the rings experience a central impact. In this case, the second ring passes through the first and decreases in size, while the first, on the contrary, expands. As in previous cases, this occurs due to the mutual action of water flows from one ring to another. Subsequently, the rings begin to divide.

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A drop of liquid can spread over a surface if the surface is well wetted, but if the surface is poorly wetted, the drop will not spread.  

A drop of liquid on the surface solid, can either spread into a thin film or remain on the surface in the form of a lens.  

A drop of liquid applied to a solid surface does not immediately form a contact angle of a constant value on it.  

A drop of liquid spreads on a solid surface under the influence of the attraction of liquid molecules to a solid molecule, including along the perimeter of the drop at a distance of action of molecular forces, as well as under the influence of gravity. The cohesive forces of attraction of liquid molecules to each other prevent spreading. The spreading of oils with additives on metal surfaces often occurs in several stages.  

A drop of liquid (by adding a drop of water and cooling) is mixed with a solution of diphenylamine in concentrated sulfuric acid.  

A drop of liquid A is placed on a silver coin. The rapid appearance of a brown-black stain that cannot be washed off with water indicates the presence of sulfur.  

A small drop of liquid containing the cell is placed on the ground edge of the chamber's capillary and the cell is observed using a microscope. If the cell floats freely in the liquid, as in the case of Paramecium, then it can be quickly introduced into the tube; otherwise, it should be introduced into the capillary using a thin needle. After the cell is introduced into the capillary, the water or liquid in which the cell was located is wiped off and a cover glass smeared with a thin layer of Vaseline is used to close the opening of the chamber. Place the tube in the water of the inner Dewar cup and fill the cup again with the solution. caustic soda. After approximately an hour, temperature equilibrium has been reached, remove the alkali solution from the cup with a thin pipette and wipe it with a cotton swab. After another hour, the meniscus is introduced into the field of view of the microscope. The magnification of the microscope should be such that approximately 100 capillary diameters fit into the field of view. An eyepiece micrometer must be inserted into the microscope eyepiece. The speed of movement of the meniscus is observed and recorded in units of micrometric scale division. During the measurement process, temperature and pressure are periodically monitored; if they change noticeably, the measurement results are considered unreliable and discarded.  

If a drop of liquid is formed as a result of gas injection, then the turbulence arising inside it is so great that the diffusion resistance of its surface layer turns out to be very small. The use of the injection principle allows the absorption process to be carried out with a high degree of intensity.  

Why does a drop of liquid tend to be spherical?  

If a drop of liquid is placed in a turbulent flow of a liquid that does not mix with it, then its fragmentation occurs under the influence of turbulent pulsations. In this case, large-scale pulsations, which change relatively little over distances of the order of the size of the drop, do not affect it; deformation and crushing are produced by small-scale pulsations. The crushing effect largely depends on what is in turbulent flow the velocity of the external phase liquid at the surface of the globules at its two points will be different.  

If a drop of liquid rests on a surface that is not wetted by this liquid, then it is flattened under the influence of gravity. However, surface tension keeps the droplet from flattening indefinitely, since flattening means increasing the surface area.  

If a drop of liquid is placed on the surface of another, immiscible liquid or solid, it can either spread or remain as a non-spreading drop. This depends entirely on the surface tensions of both liquids and on the interfacial tension between them; the same is true if the lower phase is a solid.  

Surely you have noticed that chaotically scattered drops always have round shape. Why is the drop round?

If you look closely, you will see that the shape of the drop is not at all perfectly round. For example, if you look at raindrops from below, they appear almost flat. A perfect ball is possible only in conditions of weightlessness. And since we are on Earth, the drop (like all bodies on our planet) is exposed to gravity. This makes it slightly flattened. Therefore, the shape of the drop is more likely not a sphere, but an ellipsoid, although with a very small interfocal distance.

What other force, besides the force of attraction, acts on the drop? Surface tension force. To explain how it works, let's look at the course molecular physics. The surface of a drop can be considered as a film consisting of molecules, and its molecules outer layers not in equal conditions with internal molecules. The molecules of the outer layer of the film have greater free energy. Trying to release excess energy and trying to penetrate the inner layers of the drop, they create pressure. The pressure force vector is always directed towards the center of the drop. And the force with which the molecules of the outer layers of the drop press on the molecules of the inner layers is called surface tension force.

Thus, the smaller the drops, the more round they are - they are collected into a ball by the force of surface tension. But larger drops have an elongated shape, because they are too heavy and this force is no longer enough to hold them in the shape of a ball.

But the question remains open: why is it still spherical? The above theory does not fully explain this. The fact is that on a spherical surface all the molecules located on it are in equal energy state. In other words, the spherical surface is the most energetically stable, since this is the position that is most beneficial for the system. In general, a ball is the most compact shape in nature.

If the droplet is stretched, the molecules located in the stretched areas acquire higher excess energy. In an effort to release excess energy, the molecules return the drop to its original state, which ultimately brings the system into equilibrium.

As follows from the above, surface tension seems to hold water in an elastic “skin” - a shell. This shell makes a drop hang at the end water tap. If the drop becomes too large, the shell cannot withstand it, breaks, and the drop falls.

It is thanks to the force of surface tension that the tiny insect water strider can walk on the surface of the water without plunging into it. And the basilisk lizard can calmly run across a river or small lake right on the surface of the water.

Is it possible to make a drop of water flat? Yes, and very simple. You need to gently touch it with the tip of a soapy straw. The drop becomes flat because the soap weakens the surface tension of the water - and its force is no longer enough to hold the drop in the shape of a ball.

How are soap bubbles made? When we add soap to water, the force of surface tension decreases, and the surface of the water seems to stretch and become more elastic - so elastic that you can blow air into it and it will stretch into a bubble. It's a little like filling a balloon with water.

Thus, a drop of water is not round, but ellipsoidal. The shells of various liquids have varying degrees strength. For example, alcohol has lower surface tension than water, so it forms smaller droplets. Mercury, on the other hand, has a surface tension 6 times greater than that of water, so when a thermometer breaks, it breaks up into many small balls.