Why are small drops of liquid spherical



A drops is a self-contained liquid / gaseous or liquid / liquid phase interface. If it is a liquid / liquid phase interface, there must be two immiscible liquids between which this interface can develop. In equilibrium, the bubble is spherical due to the surface tension γ, since the surface A.O is decreased to the surface energy

to reduce.

Due to the influence of surface tension, the drop strives to acquire a spherical shape. However, this is prevented by its toughness, inertia, air friction and other external forces, so that it deforms into a spherical shape via a slightly oscillating drop.

The complement to the drop is the bubble.

Teardrop shape

Designated in common parlance Teardrop shape a two- or three-dimensional shape that is round on one side and tapers to a point on the other. Contrary to popular belief, a drop never has a “drop shape”.

Drop formation

 

When a drop begins to form, a constriction occurs as expected. But instead of simply tapering further, so that a "teardrop shape" would arise, it is elongated. The result is a thread-like structure, at the end of which an almost spherical drop hangs. Where the “thread” meets the drop, a constriction is formed again. If the viscosity of the liquid is high enough (higher than that of water), this constriction will also lengthen. The higher the viscosity, the more often this process is repeated. At some point, however, this becomes unstable and the drop becomes detached from the thread. Other, smaller drops sometimes form from the thread.

The detachment of a drop can be easily observed on a lava lamp without any further technical aids.

Raindrops

Rain is a form of precipitation, i.e. condensed water vapor. Raindrops do not have a “drop shape” either. With a droplet size of up to 0.5 mm, they are spherical. Normal raindrops 2-3 mm in diameter are hemispherical at the top and dented at the bottom due to air resistance. As an intermediate stage, there are drops that are flattened at the bottom. Large drops from thunderstorm rain (max. 9 mm) become unstable and tear through the air resistance. With a drop radius of 0.05 to 0.25 mm one speaks of drizzle.

The droplet size within the precipitation is statistically distributed, whereby a respective maximum can be assigned to different rain intensities.

print

The internal drop pressure p depends on the surface tension (or general interfacial tension) γ of the liquid (actually the liquid / gas interface) and the radius r, as well as the air pressure. Strictly speaking, p is the difference between the capillary curvature pressure pK and the external static pressure ps. The capillary curvature pressure results in

.

Small drops therefore have a high internal pressure. If the drop is not round, the two mutually perpendicular and extreme radii must be used r1 and r2 of the UI element on which pK acts, looks at and sustains

.

Swinging drops of water

A drop of water, which is set in oscillation at a frequency between 30 and 120 Hertz, can create geometrical, plastic shapes at the right temperature, air pressure, and many other factors. The up and down movement of the carrier plate makes the water "dance".

See also

literature

  • Ian Stewart: The teardrop shape In: Spectrum of science 03/1996, Spektrum der Wissenschaft Verlag, page 10, ISSN 0170-2971

Category: Soft Matter