Editing Drop (liquid) (section)

== Shape ==
[[File:Raindrops sizes.svg|thumb|Raindrops are not tear-shaped (Ⓐ); very small raindrops are almost spherical in shape (Ⓑ), while larger raindrops are flattened at the bottom (Ⓒ). As raindrops increase in size they encounter progressively more air resistance as they fall, making them begin to become unstable (Ⓓ); in the case of the largest raindrops, air resistance will be enough to split them into smaller raindrops (Ⓔ).|188x188px]]
The classic shape associated with a drop (with a pointy end in its upper side) comes from the observation of a droplet clinging to a surface. The shape of a drop falling through a gas is actually more or less spherical for drops less than 2&nbsp;mm in diameter.<ref name="Pruppacher 1971 86–94">{{cite journal
| last = Pruppacher
| first = H. R.
| author2 = Pitter, R. L.
| year = 1971
| title = A Semi-Empirical Determination of the Shape of Cloud and Rain Drops
| journal = Journal of the Atmospheric Sciences
| volume = 28
| issue = 1
| pages = 86–94
| doi = 10.1175/1520-0469(1971)028<0086:ASEDOT>2.0.CO;2   |bibcode = 1971JAtS...28...86P | doi-access = free
}}</ref> Larger drops tend to be flatter on the bottom part due to the pressure of the gas they move through.<ref>{{cite web|url=http://www.newton.dep.anl.gov/askasci/gen01/gen01429.htm|title=Water Drop Shape|access-date=2008-03-08|archive-date=2008-03-02|archive-url=https://web.archive.org/web/20080302030110/http://www.newton.dep.anl.gov/askasci/gen01/gen01429.htm|url-status=dead}}</ref> As a result, as drops get larger, a concave depression forms which leads to the eventual breakup of the drop.

=== Capillary length ===
The [[capillary length]] is a length scaling factor that relates [[gravity]], density, and [[surface tension]], and is directly responsible for the shape a droplet for a specific fluid will take. The capillary length stems from the [[Laplace pressure]], using the radius of the droplet.

Using the capillary length we can define microdrops and macrodrops. Microdrops are droplets with radius smaller than the capillary length, where the shape of the droplet is governed by surface tension and they form a more or less [[spherical cap]] shape. If a droplet has a radius larger than the capillary length, they are known as macrodrops and the gravitational forces will dominate. Macrodrops will be 'flattened' by gravity and the height of the droplet will be reduced.<ref>{{Cite book|title=Microfluidics for biotechnology|last=Berthier|first=Jean|date=2010|publisher=Artech House|others=Silberzan, Pascal.|isbn=9781596934443|edition= 2nd|location=Boston|oclc=642685865}}</ref>
[[File:Sessile drop Capillary Length.jpg|center|thumb|389x389px|The capillary length <math>L_c</math> against radii of a droplet|alt=]]