# Tag Info

56

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 altered ...

38

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. ...

37

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 ...

28

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

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 ...

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 ...

20

Yes, as you can see in this video. As you can see, the droplet will hit the surface, partially coalesce (merge) with the bulk, re-emerge as a smaller droplet, bounce 1-3 times, partially coalesce again, re-emerge again as an even smaller droplet and so on. This process is known as coalescence cascade. You can find another video here. Eventually, the ...

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 ...

16

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 http://www.telegraph.co.uk/science/science-news/6454568/How-to-stop-a-teapot-...

16

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 ...

16

Let's throw some numbers at this. The Eotvos (or Bond) number is a dimensionless ratio of the body forces to surface tension forces often used in the sciences to characterize certain flows regimes. This number is given by: $$\mathrm{Eo}=\frac{\Delta\rho g L^2}{\sigma}$$ where $\Delta\rho$ is the density differences between two phases, $g$ is the ...

15

The gravitational binding energy for a spherical object of mass $M$ and radius $R$ is given by: $$E_{grav}=\frac35 \frac{GM^2}{R}$$ The interfacial energy for a spherical droplet is simply proportional to its surface area: $$E_{surf}=4\pi \sigma R^2$$ Here $\sigma$ denotes the droplet's surface tension. Taking the ratio of the two energies and using $M = \... 14 This is due to surface tension. Water wants to stick to hard surfaces as this is a lower energy arrangement. Component of gravity perpendicular to glass pulls water away from glass wall, and surface tension pulls water to glass wall. When the angle between glass wall and vertical direction is small, component of gravity perpendicular to glass wall is ... 14 I looked up the answer to this one in a book published in 1914 - you don't get many citations 99 years old! For the interested, the book is "A Textbook of Physics Vol 1" by J. H. Poynting and J. J. Thompson, page 188 in my copy. Incidentally that's the same J. J. Thompson who discovered the electron - Poynting has a vector named after him though only ... 14 The problem you have is surface tension. The drop will continue to grow until the weight of the drop is large enough that the "cost" of increasing the drop's surface by A (the contact area of the drop with the orifice) is less than the gain in energy from falling. This is described in some detail in this wikipedia entry. This describes how drop size in the ... 14 The shape of the curve must be such that the pressure difference across the meniscus exactly counters the force of gravity on the additional column of water. Now if we write the height of the liquid (density$\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 ...

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 ...

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 ...

12

This is because that the cork floats to the highest point it can. The water is not flat in the glass - it curves up a the edges so the cork gets higher by going to the edge. When the glass is full, really completely full just above the level of the rim of the glass, the water will be bowed a bit so that it goes down at the edges and is highest in the ...

10

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 ...

10

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

Because liquids, water in particular have a high surface tension. For this reason blobs of water tend to become spherical. Now, given that it starts as an elongated stream (say because it's pushed out of a bottle), the stream breaks up in different pseudo-spherical bubbles; if an astronaut were to pour water very, very slowly and carefully he could create ...

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

Surface tension occurs because water molecules attract on another. That means that water prefers to form compact shapes with little surface area. Creating a large, extended area, as you do in a soap bubble, is actually opposed by surface tension. This is why you can't get stable bubbles with pure water: the bubble wants to collapse into a compact shape. ...

6

I know this is an old question, but for the benefit of people visiting here wondering what the answer was, here it goes: A droplet can stay at rest on an inclined plate because of small heterogeneities on the surface. This can either be a small roughness (of the order of nano/micrometers) or `dirty' spots where the surface chemistry is locally different. ...

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

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 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, vorticity-...

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 ...

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