Editing Surface tension (section)

==Methods of measurement==

Because surface tension manifests itself in various effects, it offers a number of paths to its measurement. Which method is optimal depends upon the nature of the liquid being measured, the conditions under which its tension is to be measured, and the stability of its surface when it is deformed. An instrument that measures surface tension is called [[Tensiometer (surface tension)|tensiometer.]]

* [[Du Noüy ring method]]: The traditional method used to measure surface or interfacial tension. Wetting properties of the surface or interface have little influence on this measuring technique. Maximum pull exerted on the ring by the surface is measured.<ref name="phywe"/>
*[[Wilhelmy plate|Wilhelmy plate method]]: A universal method especially suited to check surface tension over long time intervals. A vertical plate of known perimeter is attached to a balance, and the force due to wetting is measured.
* [[Spinning drop method]]: This technique is ideal for measuring low interfacial tensions. The diameter of a drop within a heavy phase is measured while both are rotated.
* [[Pendant drop method]]: Surface and interfacial tension can be measured by this technique, even at elevated temperatures and pressures. Geometry of a drop is analyzed optically. For pendant drops the maximum diameter and the ratio between this parameter and the diameter at the distance of the maximum diameter from the drop apex has been used to evaluate the size and shape parameters in order to determine surface tension.
* [[Bubble pressure method]] (Jaeger's method): A measurement technique for determining surface tension at short surface ages. Maximum pressure of each bubble is measured.
* Drop volume method: A method for determining interfacial tension as a function of interface age. Liquid of one density is pumped into a second liquid of a different density and time between drops produced is measured.
* Capillary rise method: The end of a capillary is immersed into the solution. The height at which the solution reaches inside the capillary is related to the surface tension by the equation [[#Liquid in a vertical tube|discussed above]].<ref name="calvert">{{cite web|url=http://mysite.du.edu/~jcalvert/phys/surftens.htm|title=Surface Tension (physics lecture notes)|author=Calvert, James B|access-date=2007-09-08|publisher=University of Denver|archive-date=2007-09-15|archive-url=https://web.archive.org/web/20070915112348/http://mysite.du.edu/~jcalvert/phys/surftens.htm|url-status=live}}</ref>
* [[Stalagmometric method]]: A method of weighting and reading a drop of liquid.
* [[Sessile drop method]]: A method for determining surface tension and [[density]] by placing a drop on a substrate and measuring the [[contact angle]] (see [[Sessile drop technique]]).<ref name="dp">{{cite web|url=http://www.dataphysics.de/english/messmeth_sessil.htm|title=Sessile Drop Method|access-date=2007-09-08|publisher=Dataphysics |archive-url = https://web.archive.org/web/20070808082309/http://www.dataphysics.de/english/messmeth_sessil.htm |archive-date = August 8, 2007}}</ref>
*[[Du Noüy–Padday method]]: A minimized version of Du Noüy method uses a small diameter metal needle instead of a ring, in combination with a high sensitivity microbalance to record maximum pull. The advantage of this method is that very small sample volumes (down to few tens of microliters) can be measured with very high precision, without the need to correct for [[buoyancy]] (for a needle or rather, rod, with proper geometry). Further, the measurement can be performed very quickly, minimally in about 20 seconds.
* Vibrational frequency of levitated drops: The natural frequency of vibrational oscillations of magnetically levitated drops has been used to measure the surface tension of superfluid <sup>4</sup>He. This value is estimated to be 0.375&nbsp;dyn/cm at {{mvar|T}} = 0&nbsp;K.<ref>{{cite journal|doi=10.1103/PhysRevB.66.214504|title=Surface tension of liquid 4He as measured using the vibration modes of a levitated drop|year=2002|last1=Vicente|first1=C.|last2=Yao|first2=W.|last3=Maris|first3=H.|last4=Seidel|first4=G.|journal=Physical Review B|volume=66|issue=21|pages=214504|bibcode = 2002PhRvB..66u4504V }}</ref>
* Resonant oscillations of spherical and hemispherical liquid drop: The technique is based on measuring the resonant frequency of spherical and hemispherical pendant droplets driven in oscillations by a modulated electric field. The surface tension and viscosity can be evaluated from the obtained resonant curves.<ref>{{cite journal | doi = 10.1016/j.colsurfa.2013.12.013 | volume=460 | title=Droplet oscillations driven by an electric field | journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects | pages=351–354 | year=2014 | last1 = Zografov | first1 = Nikolay}}</ref><ref>{{cite journal | doi = 10.1524/zpch.2013.0420 | volume=227 | issue=12 | title=Electrically Driven Resonant Oscillations of Pendant Hemispherical Liquid Droplet and Possibility to Evaluate the Surface Tension in Real Time | journal=Zeitschrift für Physikalische Chemie | year=2013 | last1 = Tankovsky | first1 = N.| pages=1759–1766 }}</ref><ref>{{cite journal | doi = 10.1524/zpch.2011.0074 | volume=225 | issue=4 | title=Oscillations of a Hanging Liquid Drop, Driven by Interfacial Dielectric Force | journal=Zeitschrift für Physikalische Chemie | pages=405–411 | year=2011 | last1 = Tankovsky | first1 = Nikolay }}</ref>
* Drop-bounce method: This method is based on aerodynamic levitation with a split-able nozzle design. After dropping a stably levitated droplet onto a platform, the sample deforms and bounces back, oscillating in mid-air as it tries to minimize its surface area. Through this oscillation behavior, the liquid's surface tension and viscosity can be measured.<ref>{{cite journal |last1=Sun |first1=Yifan |last2=Muta |first2=Hiroaki |last3=Ohishi |first3=Yuji |title=Novel Method for Surface Tension Measurement: the Drop-Bounce Method |journal=Microgravity Science and Technology |date=June 2021 |volume=33 |issue=3 |pages=32 |doi=10.1007/s12217-021-09883-7|bibcode=2021MicST..33...32S |doi-access=free }}</ref>