Currently, the stainless steel needles and the teflon needles are the most common used in the optical contact angel meters. Compared with non-contact jet needle (the virtual needles or the so called liquid needles), the classical needles need to move down to let the droplet come into contact with the sample surface and then move up to leave the droplet stay on the substrate during the liquid transition part. The defects of the classical needles are mainly reflected in three aspects:
1. Drop deposition on super-hydrophobic surfaces is very difficult. Generally, the droplet could not be transferred to the substrate. This problem is mainly due to the surface free energy of the super-hydrophobic surface is much lower than that of the needle (stainless steel or PFTE). At that time, surface free energy of solid was unable to overcome the energy of the needle, so the droplet would be away from the super-hydrophobic surface and stay on the top of the needle. The transfer process is very difficult especially for the materials with very limited surface area such as an area of only 2*2 mm or the fiber type (Ф1-2mm or thinner) super-hydrophobic materials. It is unable to realize the liquid transfer.
2. It is unable to transfer the droplet on a very small area such as the range of nanoscale. It is difficult to control the droplet size accurately. Min volume of jet needle is 1nL or 5pL (Depends the system you choosing).
3. It could not transfer the liquid sample with high viscosity (max 2000.mpa.s).
Our jet needles or liquid needles could solve these problems perfectly. As the liquid transfer is using the jet method instead of the traditional drop transfer method, all of the above issues would not happen. On the other hand, because the minimum amount of the liquid feed could be as low as 5nL, 1nL or even lower (5pL, but this system is not suitable for high viscosity sample), this technology enhances the scope of the application of the optical contact angle measurement greatly. Particularly, for the investigation in the chip, semiconductor, super-hydrophobic fiber material and graphene material, this technology would play a very important part. While using this technology, it must be assisted with another accessory—the micrometer-scale optical level adjustment sample stage. Because when measuring the super-hydrophobic materials, an extra ejection force which caused by the jet needle would make the droplets scroll. In this situation a very good levelness sample substrate is necessary.
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