3.6.3 Experimental principles
Liquid surface tension refers to a force acting to reduce the surface area of the liquid because the molecules at the surface of a liquid experience a net attraction toward the interior of the liquid.The liquid surface tension coefficient is an important physical quantity to describe the properties of a liquid,such as foam formation,wetting and capillary phenomena.Many methods,including Du Nouy ring method,Wilhelmy plate method,drop method and bubble pressure method,have been employed to measure the surface tension coefficient of a liquid.In this experiment,a simple method,namely the pull-out method,has been used to determine the cofficient of the surface tension of distilled water and absolute ethanol.The calculation of the surface tension coefficient is based on the measurement of the maximum force and the perimeter of the ring.
In this experiment,the lifting ring is hung on the measurement hook connected to the silicon piezoresistive force sensor.The tension exists on the surface layer once the ring contacts the liquid.Under the first order approximation,the strength of tension is directly proportional to the length of the straight line,which can be expressed as:
Where f is the surface tension of the liquid,and L is the contact length between the lifting ring and the liquid surface,and the proportional coefficient α is called the surface tension coefficient.
When the lifting ring is slowly moved out of the liquid with a constant speed,a liquid meniscus is pulled out by the ring.The total force needed to detach the ring equals to the ring weight and the surface tension,therefore,it can be expressed as:
Where f1 represents the pull force of silicon piezoresistive force sensor,D1 and D2 are the outer diameter and inner diameter of the ring(D1=34.96 mm,D2=33.10 mm),and m is the mass of the ring,while g is gravity acceleration.Then,the tension of the sensor is proportional to the output voltage of the force sensor,which can be written as,
Where k is the sensitivity of the force sensor and can be measured in the experiment.(https://www.daowen.com)
After the ring is lifted off,the tensile force f2 on the sensor is:
For the same reason,it can be written as follows:
Therefore,the variation of the voltage output before and after the ring lifting off can be expressed as follows:
So the surface tension coefficient of liquid is:
