View source for Drop (liquid)

{{Redirect2|Droplet|raindrop|other uses|Droplets (disambiguation)|and|Raindrops (disambiguation)}}

[[Image:Water drop animation enhanced small.gif|thumb|Water drops falling from a tap.]]
[[File:Water droplet surviving an attempt to be cut by a knife.ogv|thumb|Surface tension prevents the droplet from being cut by a knife]]
[[File:RainDrops1.jpg|thumb|Rain water flux from a canopy. Among the forces that govern drop formation: [[Surface tension]], [[Cohesion (chemistry)]], [[Van der Waals force]] , [[Plateau–Rayleigh instability]].]]

A '''drop''' or '''droplet''' is a small column of [[liquid]], bounded completely or almost completely by [[free surface]]s. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a [[pendant]] drop. Drops may also be formed by the [[condensation]] of a [[vapor]] or by [[atomizer nozzle|atomization]] of a larger mass of liquid.

== Surface tension ==
[[Image:Bouncing droplets compact size.gif|left|120px|thumb|Drop of water dancing on a water surface subject to vibrations]]
[[Image:Pendant drop test.svg|thumb|120px|The pendant drop test illustrated.]]
<!-- Image with unknown copyright status removed: [[Image:Pearl_droplet.jpg|thumb|right|Droplet frozen by flash. www.liquidartgallery.com]] -->

Liquid forms drops because the liquid exhibits [[surface tension]].<ref>{{Cite book|url=https://books.google.com/books?id=o8MdoOd6pOcC&pg=PA196|title=The American Desk Encyclopedia|last=Luck|first=Steve|date=1998|publisher=Oxford University Press, USA|isbn=978-0-19-521465-9|page=196}}</ref>

A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension.

=== Viscosity and pitch drop experiments===
{{main|Viscosity|Pitch drop experiment}}
Some substances that appear to be solid, can be shown to instead be extremely [[viscosity|viscous]] liquids, because they form drops and display droplet behavior. In the famous [[pitch drop experiment]]s, [[pitch (resin)|pitch]] – a substance somewhat like solid [[bitumen]] – is shown to be a liquid in this way. Pitch in a funnel slowly forms droplets, each droplet taking about 10 years to form and break off.

== Pendant drop test ==
In the pendant drop test, a drop of liquid is suspended from the end of a tube by [[surface tension]]. The force due to surface tension is proportional to the length of the boundary between the liquid and the tube, with the proportionality constant usually denoted <math>\gamma</math>.<ref>{{cite book
  | last = Cutnell
  | first = John D.
  |author2=Kenneth W. Johnson
  | title = Essentials of Physics
  | publisher = Wiley Publishing
  | year = 2006 }}</ref> Since the length of this boundary is the circumference of the tube, the force due to surface tension is given by

: <math>\,F_{\gamma} = \pi d \gamma</math>

where ''d'' is the tube diameter.

The mass ''m'' of the drop hanging from the end of the tube can be found by equating the force due to gravity (<math>F_{g} = mg</math>) with the component of the surface tension in the vertical direction (<math>F_{\gamma} \sin \alpha</math>) giving the formula

: <math>\,mg = \pi d \gamma \sin \alpha</math>

where α is the [[contact angle|angle of contact]] with the tube, and ''g'' is the acceleration due to gravity.

The limit of this formula, as α goes to 90°, gives the maximum weight of a pendant drop for a liquid with a given surface tension, <math>\gamma</math>.

: <math>\,mg = \pi d \gamma</math>

This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry. More sophisticated methods are available to take account of the developing shape of the pendant as the drop grows. These methods are used if the surface tension is unknown.<ref>{{Cite journal
  | author = Roger P. Woodward, Ph.D.
  | title = Surface Tension Measurements Using the Drop Shape Method
  | publisher = First Ten Angstroms
  | url = http://www.firsttenangstroms.com/pdfdocs/STPaper.pdf
  | format = PDF
  | accessdate = 2008-11-05}}</ref><ref>{{cite journal
 | author = F.K.Hansen
 |author2=G. Rodsrun
 | year = 1991
 | title = Surface tension by pendant drop. A fast standard instrument using computer image analysis
 | journal = Colloid and Interface Science
 | volume = 141
 |issue=1
 | pages = 1–12
 | doi = 10.1016/0021-9797(91)90296-K
 | bibcode = 1991JCIS..141....1H
 }}</ref>

== Drop adhesion to a solid ==
The drop [[adhesion]] to a solid can be divided into two categories: lateral adhesion and normal adhesion. Lateral adhesion resembles friction (though [[tribology|tribologically]] lateral adhesion is a more accurate term) and refers to the force required to slide a drop on the surface, namely the force to detach the drop from its position on the surface only to translate it to another position on the surface. Normal adhesion is the adhesion required to detach a drop from the surface in the normal direction, namely the force to cause the drop to fly off from the surface. The measurement of both adhesion forms can be done with the [[Centrifugal Adhesion Balance]] (CAB). The CAB uses a combination of centrifugal and gravitational forces to obtain any ratio of lateral and normal forces. For example, it can apply a normal force at zero lateral force for the drop to fly off away from the surface in the normal direction or it can induce a lateral force at zero normal force (simulating zero [[gravity]]).

== Droplet ==
The term '''droplet''' is a diminutive form of 'drop' – and as a guide is typically used for liquid particles of less than 500&nbsp;μm diameter.  In [[pesticide application|spray application]], droplets are usually described by their perceived size (i.e., diameter) whereas the dose (or number of infective particles in the case of [[biopesticide]]s) is a function of their volume.  This increases by [[Volume of a sphere|a cubic function]] relative to diameter<!-- (π.d<sup>3</sup>/6000 to convert µm into picolitres)-->; thus a 50&nbsp;μm droplet represents a dose in 65&nbsp;pl and a 500&nbsp;μm drop represents a dose in 65 nanolitres.

== Speed ==
A droplet with a diameter of 3&nbsp;mm has a terminal velocity of approximately 8&nbsp;m/s.<ref name="uvasim"/>
Drops smaller than {{nowrap|1 mm}} in diameter will attain 95% of their terminal velocity within {{nowrap|2 m}}. But above this size the distance to get to terminal velocity increases sharply. An example is a drop with a diameter of {{nowrap|2 mm}} that may achieve this at {{nowrap|5.6 m}}.<ref name="uvasim">{{cite web |title=Numerical model for the fall speed of raindrops in a waterfall simulator |url=http://staff.science.uva.nl/~jboxel/Publications/PDFs/Gent_98.pdf |page=2 |date=2005-10-04 |accessdate=2013-06-28 |url-status=dead |archiveurl=https://web.archive.org/web/20130731002444/http://staff.science.uva.nl/~jboxel/Publications/PDFs/Gent_98.pdf |archivedate=2013-07-31 }}</ref>

== Optics ==
Due to the different [[refractive index]] of [[water]] and [[Earth's atmosphere|air]], [[refraction]] and [[reflection (physics)|reflection]] occur on the surfaces of [[rain]]drops, leading to [[rainbow]] formation.

== Sound ==
{{listen|filename=Water drops dripping.ogg|title=Drops dripping into water.}}

The major source of sound when a droplet hits a liquid surface is the [[Liquid bubble#Pulsation|resonance of excited bubbles]] trapped underwater.  These oscillating bubbles are responsible for most liquid sounds, such as running water or splashes, as they actually consist of many drop-liquid collisions.<ref>{{cite journal
 | last = Prosperetti
 | first = Andrea
 | authorlink = Andrea Prosperetti
 |author2=Oguz, Hasan N.
 | year = 1993
 | title = The impact of drops on liquid surfaces and the underwater noise of rain
 | journal = Annual Review of Fluid Mechanics
 | volume = 25
 | pages = 577–602
 | doi = 10.1146/annurev.fl.25.010193.003045
 |bibcode = 1993AnRFM..25..577P }}</ref><ref>{{cite web |url=http://ffden-2.phys.uaf.edu/311_fall2004.web.dir/Ryan_Rankin/bubble%20resonance.htm |title=Bubble Resonance |accessdate=2006-12-09 |last=Rankin |first=Ryan C. |date=June 2005 |website=The Physics of Bubbles, Antibubbles, and all That}}</ref>

==="Dripping tap" noise prevention===
Reducing the surface tension of a body of liquid makes possible to reduce or prevent noise due to droplets falling into it.<ref>{{cite web|url=https://mashable.com/2018/06/25/dripping-tap-sound-solved|title=Scientists have finally come up with a solution for the world's most annoying household sound|first=Rachel|last=Thompson|publisher=}}</ref> This would involve adding [[soap]], [[detergent]] or a similar substance to water. The reduced surface tension reduces the noise from dripping.

== Shape ==
[[File:Raindrops sizes.svg|thumb|The shapes of raindrops, depending on their sizes.|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 }}</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|accessdate=2008-03-08}}</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]] 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 solely by surface tension and they form a spherical cap shape. If a droplet has a radii 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|url=https://www.worldcat.org/oclc/642685865|title=Microfluidics for biotechnology|last=1952-|first=Berthier, 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=]]

== Size ==
Raindrop sizes typically range from 0.5&nbsp;mm to 4&nbsp;mm, with size distributions quickly decreasing past diameters larger than 2-2.5&nbsp;mm.<ref>{{Cite book
| last = McFarquhar
| first = Greg
| year = 2010
| title = Raindrop Size Distribution and Evolution
| journal = Geophysical Monograph Series
| volume = 191
| pages = 49–60
| doi = 10.1029/2010GM000971|bibcode = 2010GMS...191...49M | isbn = 978-0-87590-481-8
}}</ref>

Scientists traditionally thought that the variation in the size of raindrops was due to collisions on the way down to the ground. In 2009 French researchers succeeded in showing that the distribution of sizes is due to the drops' interaction with air, which deforms larger drops and causes them to fragment into smaller drops, effectively limiting the largest raindrops to about 6&nbsp;mm diameter.<ref>{{cite journal |author=Emmanuel Villermaux, Benjamin Bossa |title=Single-drop fragmentation distribution of raindrops |url=https://www.irphe.fr/~fragmix/publis/VB2009.pdf |journal=Nature Physics |date=September 2009 |volume=5 |pages=697–702 |doi=10.1038/NPHYS1340 |lay-url=http://news.bbc.co.uk/2/hi/science/nature/8155883.stm |issue=9 |bibcode = 2009NatPh...5..697V }}</ref> However, drops up to 10&nbsp;mm (equivalent in volume to a sphere of radius 4.5&nbsp;mm) are theoretically stable and could be levitated in a wind tunnel.<ref name="Pruppacher 1971 86–94"/>
The largest recorded raindrop was 8.8&nbsp;mm in diameter, located at the base of a [[cumulus congestus cloud]] in the vicinity of [[Kwajalein Atoll]] in July 1999. A raindrop of identical size was detected over northern Brazil in September 1995.<ref>{{cite journal
| last = Hobbs
| first = Peter V.
| author2 = Rangno, Arthur L.
| year = July 2004
| title = Super-large raindrops
| journal = Geophysical Research Letters
| volume = 31
| issue = 13
| pages = L13102
| doi = 10.1029/2004GL020167|bibcode = 2004GeoRL..3113102H }}</ref>

=== Standardized droplet sizes in medicine===
In [[medicine]], this property is used to create [[dropper]]s and IV infusion sets which have a [[Technical standard|standardized]] [[diameter]], in such a way that 1 [[millilitre]] is equivalent to 20 [[Drop (unit)|drops]]. When smaller amounts are necessary (such as paediatrics), microdroppers or paediatric infusion sets are used, in which 1 millilitre = 60 microdrops.<ref>{{Cite web|url=https://www6.dict.cc/wp_examples.php?lp_id=1&lang=en&s=millilitre|title=https://www6.dict.cc/wp_examples.php?lp_id=1&lang=en&s=millilitre|website=www6.dict.cc|access-date=2018-08-30}}</ref>

== Gallery ==
<gallery>
Image:Blue Droplet.jpg|Blue dye being dropped in a saucer of milk.
Image:2006-02-13 Drop-impact.jpg|Impact of a drop of water.
Image:2006-01-28 drop-impact backjet.jpg|Backjet from drop impact.
Image:Water splashes 001.jpg|A drop of water hitting a metal surface/ crown formation due to splashing of droplet.
Image:Post-splash with droplets.jpg|A drop of water hitting a wet metal surface and ejecting more droplets, which become water [[Antibubble|globule]]s and skim across the surface of the water.
Image:Water drop on a leaf.jpg|A drop of water on a leaf / [[Hydrophobic effect]]/ Partial [[Wetting]].
Image:Water droplet backjet.JPG|A triple backjet after impact.
Image:Raindrop on a fern frond.jpg|Photo of a raindrop on a fern frond.
Image:2006-01-21 Detaching drop.jpg|Detaching drop.
Image:Showerheadandwaterdroplets.jpg|Water droplets forming out of a shower head. 
Image:Asteraceae03.JPG|A drop of water on an [[Asteraceae]]
Image:A small flower refracted in rain droplets.jpg|Droplets of water refracting a small flower.
Image:Water Drop on rose leaf.JPG|A raindrop on a leaf
Image:Water_Droplets_Background.JPG|Water droplets on glass.
Image:Fountain water droplets.jpg|Fountain water droplets as seen in very short exposure
Image:Water drops on rose leaf.jpg|Rain droplets on [[Rose]] plant leaf
</gallery>

==See also==
* [[Pitch drop experiment]]
* [[Rain]]

==References==

<references/>

==External links==
{{Commons category|Water droplets}}
* [http://www.liquidsculpture.com/fine_art/ Liquid Sculpture – pictures of drops]
* [https://web.archive.org/web/20080319094446/http://www.liquidartgallery.com/ Liquid Art – Galleries of fine art droplet photography]
* (Greatly varying) calculation of water waste from dripping tap: [https://web.archive.org/web/20070929164311/http://www.ourecohouse.info/forum/viewtopic.php?t=56], [http://ga.water.usgs.gov/edu/sc4.html]

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