Contact angle measurements on solid materials
Due to the used measurement instrument and the analyzed material respectively material character are the following methods known:
* Sessile Drop Method
is an optical contact angle measurement method to evaluate the punctual wettability on solid surfaces. The angle in the three-phase point between solid body, liquid and air is identified with goniometer or with optical software analyse.
* Powder Contact Angle Method
enables the measurement of contact angle and adsorption character of powders and other porous materials. The put on weight is measured by time out of it the wettability of powders, pigments, papers and textiles is calculated.
* Dynamic Wilhelmy Method
evaluates the advancing and the receding angle on solid bodies with defined geometry. The contact angle is evaluated due to the wetting force, wetting length and surface tension.
* Single Fibre Method
works with the same principle as Dynamic Wilhelmy Method. The advancing and the receding angle are evaluated on single fibres.
Drop shape analysis – feasibilities and limits
Influence of the measurement method and analysing method
The results of drop shape analysis are sensitive to changes of boundary conditions and changes of measurement procedure. The measurement instrument has to be positioned horizontal and the optical settings like focus, magnification, illumination, contrast, reflection and perspective should harmonize with the particular measurement. An essential influence has the evaluation of the three-phase point between solid body, liquid and air where the contact angle is measured. This is automatic or manual.
The drop profile is optically detected and characterized by curves which are analyzed with following methods for sessile drops:
* Ellipse Curve Fitting Method
The whole profile of a sessile drop is fitted to a general conic section equation. The derivative of this equation at the baseline gives the slope at the three-phase point of contact and therefore the contact angle.
* Curve Ruler (Tangent method)
The profile of the sessile drop in the region of the baseline is fitted to a polynomial or curvilinear equation function. From the obtained parameters the slope at the tree-phase point of contact at the baseline from which the contact angle is calculated.
* Half theta method (Height-Width Method）
It is granted that the contact angle is not affected by the absolute drop size. The relation between maximum height and width of sessile drop let specify the contact angle.
* Circle Fitting Method
This method is used for small contact angles preferably. The drop shape is fitted to a circle segment equation. With this method, the whole drop shape is fitted and not only the base line region. Therefore, the needle must not remain in the drop.
* ADSA（Young-Laplace Equation Fitting）
The drop shape is described mathematically using the Young-Laplace equation for curved interfaces. The comparison of one measurement analyzed with the different methods show value variations of 5 degrees. Normal ADSA method used in Commercial instrument is 2 generation that name Young-Laplace fitting based select plane, such as select plane method of Hansen or Spinger ( Instrument made in Germany). No company but KINO used 3 generation that truly belongs to ADSA technology.
Influence of material and ambiance
Geometry and surface character of solid samples, for example unevenness, surface finish, pores, in homogeneity of composition have a significant influence to the measurement result. It is advantageous to know the water absorption and actual moisture content at point of measure time. The gravitation affects the drop profile until a drop volume of 5 μl insignificant (Except ADSA). As bigger the sessile drop as more sensitive is it to conditions of ambiance. Temperature and air move affect respectively change the surface characters of the liquid and therefore the drop shape.
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