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54

The mechanism at play here is surface tension. The cohesion of the molecules of water is what keeps the wasp afloat. Due to this cohesion, the surface of the water behaves like a membrane and is curved inwards. The light rays that would be refracted from the perfectly flat surface are now incident at an altered angle and are reflected or refracted by ...


37

Have a look at the Wikipedia article on raindrop formation. You'll also find lots of articles on raindrop formation and growth by Googling raindrop formation or something like that. Raindrops do coalesce, but they also fragment, and the eventual size is a balance of the two processes. The fragmentation occurs because of the forces from turbulent air flow. ...


36

I drew an image to illustrate the forces at play. For any curved surface of the bubble, the tension pulls parallel to the surface. These forces mostly cancel out, but create a net force inward. This compresses the gas inside the bubble, until the pressure inside is large enough to counteract both the outside pressure, as well as this additional force from ...


24

It's a combination of two effects: buoyancy and adhesion. Buoyancy lifts the cork up as much as possible, until it displaces its own weight of water (Archimedes' principle). For this reason, the cork will seek the highest point of the water level. Because of adhesion between the water molecules and the glass, the water level is highest at the edges (the ...


21

The increased pressure is caused by the surface tension between the soap and the surrounding air. This can be seen by a simple equilibrium energy argument. The total energy of the system reads $$ E = E_i + E_o + E_s \;, $$ where $E_i$ is the energy associated with the air inside the bubble, $E_s$ is the interfacial energy, and $E_o$ denotes the energy ...


19

There is a great paper from the group of Howard Stone on this subject: Wetting of flexible fibre arrays (freely available here) They specifically study when 2 closely positioned parallel fibers (i.e. hairs) clump together due to the water droplets on the fibers. They quantatively determine when the volume of liquid is sufficiently small to cause ...


18

What seems to be happening is that capillary effects in the presence of gravity create a situation in which the cork being maximally decentralized in the glass corresponds to a minimum energy configuration. My guess is that the cork is non-wetting, and therefore surrounded by a water surface that bends down in the proximity of the cork, thereby creating a ...


15

That is a truly amazing picture! I am by no means an expert, but I have an idea. When the wasp stands on the water, it is curved down slightly. The light that hits these parts will then be bent more outwards than if it just hit regular water. This happens at every side of the circle, so the light is always bent out, and doesn't reach the bottom at those ...


14

If you simply held a cup upside down in zero gravity, the liquid ought not to pour out. However, things in zero gravity still obey Newton's laws. If you pull away the cup, the water ought to stay behind. In reality, a sudden move of the cup would create a lower pressure behind the water than in front so the air pressure would try to keep it in the cup, but ...


13

The shape of the curve must be such that the pressure difference across the meniscus exactly counters the force of gravity on the additional column of water. Now if we write the height of the liquid (density $\rho$) as a function of radial distance $h(r)$, and surface tension as $\sigma$, then we can write the force balance on an annular column with ...


13

The problem you have is surface tension. The drop will continue to grow until the weight of the drop is large enough that the "cost" of increasing the drop's surface by A (the contact area of the drop with the orifice) is less than the gain in energy from falling. This is described in some detail in this wikipedia entry. This describes how drop size in the ...


13

I looked up the answer to this one in a book published in 1914 - you don't get many citations 99 years old! For the interested, the book is "A Textbook of Physics Vol 1" by J. H. Poynting and J. J. Thompson, page 188 in my copy. Incidentally that's the same J. J. Thompson who discovered the electron - Poynting has a vector named after him though only ...


12

Water starts to fall from the clouds when the drop size reaches a critical point which depends on a lot of factors such as the strength of upward currents, but also air density and gravity acceleration. When the drops leave the cloud no more collisions take place and their size is fixed. To create a water stream we need to concentrate water from a large ...


11

This is due to surface tension. Water wants to stick to hard surfaces as this is a lower energy arrangement. Component of gravity perpendicular to glass pulls water away from glass wall, and surface tension pulls water to glass wall. When the angle between glass wall and vertical direction is small, component of gravity perpendicular to glass wall is ...


10

You would think that's an easy question, but it's not! Actually many things involving fluid mechanics are far harder than they seem. Anyhow a team of scientists at the University of Lyons in France have been working on this. See http://arxiv.org/abs/0910.3306 for their paper or ...


9

Pour? No such thing without gravity. In NASA TV (see video), I saw the prototype coffee cups. They are shaped with a sharp crease, to allow liquid to ride up the groove. More advanced product would also mix waxy and wettable surfaces to keep it stuck to the inside of the cup but not crawl over the brim, except at the sip line. The pictures are hard to ...


8

This is a great example of how nice it often is to reason about refraction stuff using Fermat's principle. Let's reduce all this to 2 dimensions. The surface tension produces something like this: Now if we want to now were a light "ray" needs to go to get from some light source, we just need to find the way that takes it the least time. Light is slower in ...


8

First of all, Marek is right that a surface tension exists only between two different materials (well, I would say between two different phases - for example water and ice). So let's rephrase the question as "Are there two phases with zero surface tension?" and elaborate a little on the answer. The surface tension is the excess free energy (technically the ...


7

No. Free-floating bubbles form essentially spherical shapes. In general, they form shapes that minimize surface area, subject to constraints such as the bubble having to enclose a fixed volume. One way to see why the bowl shape you imagine wouldn't work is to consider a small element of the surface right near the "corner" of the bowl (where the spherical ...


6

The article's preprint Mayer H. C., Krechetnikov R. "Walking with coffee: why does it spill?," Phys. Rev. E 85, 046117 (2012). is available from the UCSB site. From a glance of the article the phenomenon is not specific only to coffee. The authors make use of the next formula: The natural frequencies of oscillations of a frictionless, ...


6

Liquids are a state of matter in which the atoms or molecules are held together by chemical bonds (a difference from gases) but the bonds are weak enough for the shape to be variable (a difference from solids). That's why it is not possible to increase or decrease their volume much; the amount of energy from these chemical bonds would rapidly increase ...


6

A drop that is free falling in vacuum is spherical. This is because free falling in a gravitational field is the same thing as being at rest with no gravitational field present: the gravitational field and the acceleration cancel each other out. Rain drops falling to the earth can have various shapes depending on their size, although I am not aware that ...


6

The highest pressure in the ocean is at the bottom of the Mariana trench, where the pressure is 1,086 atmospheres. Using the online calculator for the properties on nitrogen at 4°C and 1,000 atmospheres the density comes out as 602 kg/m$^3$, which is still less than water. So a bubble of nitrogen would rise even at the deepest point in the ocean. Response ...


6

Surface tension is not a property of materials but of interfaces between two (or more) materials. It is implicit in its definition that the interface separates two kinds of materials that behave differently (otherwise the interface would be just some imaginary surface inside the one material with no physical meaning) and so there must always be some surface ...


6

On the one hand, as the liquid is not being poured very slowly, there are different velocities in the incoming liquid: it's faster at the top, and, it seems, at the back. On the other hand, the cohesive forces (resulting in particular in surface tension at the surface of the flow) are sufficient here to prevent separation of some liquid from the rest — ...


6

A bubble, while it still exists, is balanced by three factors: 1) Surface Tension of the soapy water. 2) Internal Pressure applied by the air inside the bubble on the surface. 3) Atmospheric Pressure. When any of these are imbalanced, one force is greater than the others and this causes the bubble to pop. If you're talking about why do they burst in ...


6

The formula for capillary rise that most people know is easily derived through a pressure balance between the capillary pressure and the hydrostatic pressure. The hydrostatic pressure equals $$\Delta P_h=\rho g h$$ whereas the capillary pressure is $$\Delta P_c=\frac{2\gamma}{R}=\frac{2\gamma \cos \theta}{r}$$ So balancing these we get our 'famous' ...


6

The real issue is that the cup wasn't really full so that adding anything more would make it spill. You can clearly see the the level slowly growing above the top of the cup, as would be expected due to surface tension. Eventually another coin finally exceeded the limit, and a little water spilled. There is really nothing extraordinary going on here. ...


5

As I commented, I would think that any 3D hydrodynamics code would work. The basics of hydrodynamics can be summed up in the following five equations: \begin{eqnarray} \frac{\partial \rho}{\partial t}+\nabla\cdot\rho\mathbf{v}=0 \tag{1} \\ \frac{\partial \rho\mathbf{v}}{\partial t}+\nabla\cdot\left[\rho\mathbf{v}\otimes\mathbf{v}+P\mathbb ...


5

Interestingly, the shape of the droplet you've drawn in the schematic explains it quite well. In droplets moving over a surface (at relatively low velocities) there are two forces counteracting gravity: viscous forces and surface tension forces. Viscous forces are essentially caused by the no-slip condition at the interface between the droplet and the ...



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