| Surface defects have a huge impact on the strength,fatigue life,electrical conductivity,and optical performance of materials.For example,surface defects in metal materials can cause stress concentration,reduce the service life of the material,and increase the risk of metal cracking.Surface defects in mirror materials can affect the optical performance of the material.Currently,there are two methods for characterizing surface defects: contact and non-contact detection.Contact measurements generally involve making the probe of an instrument come into contact with the sample surface.However,because the probe is always in contact with the surface during the measurement process,the probe may damage the surface of the sample,and the probe itself may also experience wear and tear.In order to avoid damage to the sample being tested,non-destructive testing techniques have been developed using light,electricity,and magnetism as media.However,some non-destructive testing methods have limitations on the materials being tested.During the motion of a micro droplet on the surface of a material,it exhibits a phenomenon known as "pinning" when it comes into contact with surface defects.This phenomenon leads to changes in the frictional force at the solid-liquid interface,which can be used to characterize surface defects by observing changes in the friction force curve.In this paper,a novel non-destructive detection method for surface defects is proposed.A selfmade device for measuring the frictional force at the solid-liquid interface is used to investigate the effects of droplet volume,velocity,and surface tension on the detection performance.Hard molds with microstructures were fabricated using photolithography.The microstructure periods were set at 300 μm,500 μm,and 550 μm,respectively.The microstructures were replicated using PDMS,and the device was used to measure the period lengths of the microstructures.A comparison between the measured values and the actual values was conducted to evaluate the accuracy of the device.The experimental results demonstrated a high level of accuracy for the device.When measuring droplets with volumes of 5 μL and 10 μL,the detection accuracy was high.However,when the droplet volume was increased to 15 μL,the detection accuracy decreased.Using smaller droplet volumes can improve the detection accuracy.Similarly,when the displacement platform moved at speeds of 0.105 mm/s and 0.201 mm/s,the detection accuracy was high.However,when the displacement platform speed increased to 0.391 mm/s,the detection accuracy decreased.Reducing the speed of the displacement platform can improve the detection accuracy.Increasing the droplet volume or the speed of the displacement platform can enhance the detection efficiency.Hydrophilic substrates with microstructures were prepared using 3D printing.The microstructure periods were set at 400 μm and 800 μm.When deionized water was used as the detection droplet,continuous detection on the substrate could not be achieved.According to Young’s equation,the addition of Ca Cl2 increased the surface tension of the detection droplet,leading to an increased contact angle on the hydrophilic substrate.Different concentrations of Ca Cl2 solution improved the detection accuracy on the hydrophilic substrate.Samples with surface coatings containing defects were prepared,and this detection method showed good performance in detecting coating defects.Superhydrophobic surfaces with low adhesion were also prepared,but the detection droplets could not exhibit pinning on their surfaces.Therefore,this detection method was not suitable for detecting defects on such surfaces.Software simulations of the droplet defect detection process revealed that when the contact angle of the droplet on the substrate exceeded 135°,this method was not suitable for detection.This detection method is suitable for detecting surface defects on flat surfaces and can be applied to the defect detection of products such as silicon wafers and optical glass.Compared to other non-destructive detection devices,this device offers fast detection speed and low cost,thereby reducing the cost of surface defect detection. |
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