# Tag Info

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 ...

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 ...

36

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. ...

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 ...

20

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

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 ...

17

There is a great paper from the group of Howard Stone on this subject: Wetting of flexible fibre arrays (freely available here, but for some reason I am not allowed to link to it normally: http://211.144.68.84:9998/91keshi/Public/File/34/482-7386/pdf/nature10779.pdf) They specifically study when 2 closely positioned parallel fibers (i.e. hairs) clump ...

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 ...

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 ...

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

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 ...

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 ...

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 ...

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 ...

7

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 ...

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

In the west the vast majority of towels are made from cotton, and cotton is basically cellulose. The surface of cellulose is fairly reactive (the bulk isn't unless you're a termite!) and will react with water to produce surface hydroxyl groups and negatively charged groups. Both of these lower the contact angle of water on the fibres and hence increase ...

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

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 ...

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

For the case that you have drawn, the behavior of the drop is actually the exact opposite of what you mention: it will move from right to left. This is caused by surface tension and the curvature of the droplet caps which creates a larger pressure in the drop at side B than at side A. To make it more quantitative. Let's assume that the funnel is ...

5

Rising bubbles of air in a liquid oftentimes are anything but spherical. These bubbles have haphazard shapes because they are rising and because they are interacting with other nearby bubbles. The combination of drag, turbulence, and mutual interactions prevents those bubbles from taking on a nice, simple spherical shape. Here's a rather non-spherical ...

5

No, it won't overflow. That should be obvious since doing so would create a constant flow, constantly using energy, but without any energy input. Put another way, that would be a perpetual motion machine, one you could actually extract free power from. The same force that pulls the water along the inside of the capillary tube also holds it there when it ...

5

Yes, water still has surface tension in a vacuum. Water/vacuum surface tension is 72.8 dyn/cm experimentally according to Zhang et al. J. Chem. Phys. 103, 10252 (1995). Surface tension is caused by the fact that water molecules in the bulk (not at the surface), are surrounded by other water molecules with which they interact through intermolecular ...

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