Drop shape analysis is based on image analytical technology, which is used for measurement of sessile drop (captive bubble method) based contact angle or pendant drop/sessile drop method based surface/interfacial tension. So, camera is obligate for contact angle measuring in drop shape analytical method based interfacial tensiometer. There are a variety of cameras with different resolutions on the market, such as 0.4 Megapixels (756*576), 1.3 Megapixels (1280*1024) and 10.1 Megapixels (3840*2748) , etc.. But which resolution do we really need for measurement of contact angle and surface tension?
There are three specs should be considered: geometry of pendant drop/sessile drop, image magnification of lens and sensor size of CCD.
(1)The largest or smallest geometry of pendant drop or sessile drop is usually limited. In order to obtain more accurate test results for pendant drop/sessile drop, volume of drop or bubble should be between 0.1-30uL, and 2-10uL is the best. About image magnification of lens in this situation, usually, it is about 0.2-4.5X optical magnification for currently commercial contact angle meter or surface tensiometer based on drop shape analytical system.
(2) To determine drop or bubble geometry and image magnification of lens, resolution of image is usually fixed, such as 500*500.
(3) The last problem is to determine the resolution of camera, in which situation sensor size of CCD should be taken into consideration, as small sensor size sometimes can image not under all pixels of the drop or bubble.
Experimental results show 0.4 Megapixels is the minimum resolution for measuring pendant drop or sessile drop, and sometimes, it is a little short for measuring contact angle or interfacial tension. As shown below:
Figure1: A pendant drop by camera with resolution of 0.4M; Volume is about 29uL.
Figure 2: A pendant drop by camera with resolution of 1.3M; Volume is about 29uL.
Figure 3: A pendant drop by camera with resolution of 5M; Volume is about 30uL.
Figure 4: A pendant drop by camera with resolution of 10M; Volume is about 31uL.
Too higher resolution, such as 5 Megapixels, may result in extra blank in drop image, which means the resolution is not suitable for measuring contact angle or interface tension. Camera with resolution of 1.3 Megapixels is most suitable for drop shape analysis based contact angle meter and interfacial tensiometer.
