TEST & MEASUREMENT FEATURE In contact with angle measurement Video-based contact angle measurement and drop shape analysis allows the
wetting or the adhesion behaviour of liquids on surfaces and the surface tension of liquids to be measured and analysed. Steve Hearn, IDS Imaging Development Systems, explains why high-resolution image acquisition at high frame rates is a key element of commercial drop shape analysers
T
he surface tension of liquids is important in cleaning processes, or in
printing, painting or coating – and such processes can be optimised by modifying the surface tension of the liquid. Drop shape analysis, used both in QA and R&D, uses image analysis to determine the contact angle from the shadow image of a drop of liquid on a solid surface (sessile drop), or the surface tension or interfacial surface tension from the shadow image of a drop at the tip of a needle (pendant drop). Contact angle measurement is a
measure of the wettability of a solid that is prepared for contact with a liquid; and this can be deliberately improved (e.g. for preparing for painting) or reduced (e.g. if liquids should roll off better). The camera captures an image of either the sessile drop or pendant drop, and this is transferred to image analysis software. A grey-scale analysis of the image is used for initial detection of the contour. The second step is to fit a geometrical model describing the drop shape to the contour. This contact angle is the angle between the calculated drop shape function and the contact surface. It is, however, important to capture
the dynamics of the wetting, which means that a sequence of images must be captured at high resolution and high frame rate. The cameras generally capture the wetting processes at up to 200 frames per second at a resolution of 1200 x 800 pixels. Depending on the measuring task, the resulting videos can be anywhere between a millisecond and several hours long, generating a lot of image data for analysis – which is why cameras with the high bandwidth USB 3.0 interface are chosen.
CAMERA PERFORMANCE KRÜSS is a specialist in interfacial chemistry and a leading supplier of universal drop shape analysers. Its DSA25, DSA30, and DSA100 instruments utilise USB 3.0 cameras from IDS which provide frame rates up to 200 fps at a resolution of 1200 x 800 pixels and excellent light sensitivity, using the
The software can display each drop
latest CMOS sensor technology. The UI-3060CP Rev. 2 drop shape
analyser features the IMX174 CMOS sensor from Sony and is just 29 x 29 x 29mm size, making it suited to instruments where space is at a premium. The high resolution 2.3 megapixel sensor has an analogue memory which effectively prevents focal plane distortions by simultaneous reading of all pixel signals. It also supports a dual frame output mode with various exposure times. Combining two images with different exposures increases the dynamic range to 73 dB (12 bits). High sensitivity and extremely low readout noise are especially important for contact angle measurement and the IMX174 sensor is capable of detecting just a few photons. Some applications such as the wetting of absorbent materials require even faster frame rates, and by reducing the image size to just 90 x 60 pixels, a frame rate of up to 2000 frames per second can be achieved with these cameras.
HIGH SPEED DATA TRANSFER Since the contact angle is being measured continuously, all frames from the videos are used. The USB 3.0 interface allows the huge volume of image information to be transferred at high speed and without losses to the KRÜSS ADVANCE software, for fully automated analysis of the drop shape.
INSTRUMENTATION | JULY/AUGUST 2016 35
USB 3.0 industrial cameras from IDS's USB 3 uEye CP series are used in the KRÜSS drop shape analysers
Contact angle measurement is a measure of the wettability of a solid that is prepared for contact with a liquid
image with its corresponding measured value. The image can be re-analysed at any time using different parameters or methods without time-consuming saving, loading and management of image files. Continuous background recording of the camera image takes place even when no measurement is in progress. The live image can therefore be stopped at any time to analyse the previous period or to save it as a video file. This means that valuable information cannot be lost – for example, if no measurement has been started at the time of wetting. The camera is integrated into the KRÜSS image analysis software using the uEye API, which is part of the IDS Software Development Kit (SDK) supplied with all IDS cameras. This supports all the features of modern CMOS sensors and includes an SDK for Windows and Linux (32 and 64-bit), and is the same for all camera models, whether they have a USB 3.0, USB 2.0 or GigE connection. As a result, cameras can be very quickly and easily integrated into the system and can be readily interchanged if required as new models become available, by simply adjusting camera-specific parameters with no further reprogramming.
IDS Imaging Development Systems T: 0118 970 1704
www.ids-imaging.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64