技术文章 / Technical articles
前进角、后退角的测试也可以称为动态接触角测试，其用以表征固体材料表面粗糙度对接触角测值影响，可以表征化学多样性、异构性等，所以，前进角、后退角的测试是接触角滞后现象的一个很好的测试工具。通常而言，我们把大的接触角称为前进角（ θa），把小的接触角称为后退角（θr）而把前进、后退角的差异值称为滞后接触角（ θhyst）
显然：θhyst ＝θa－θr (1.21)
虽然近几十年来一直有科学家在研究接触角滞后现象，但由于测试技术等方面原因，一直没有找到一个可以让大部分研究人员信任的可靠的测试手段去表征接触角滞后现象。接触角滞后的关键研究包括了材料的表面粗糙度，化学多样性以及亚稳定状态的表面能量变化。有些研究发现，接触角滞后角会因单分子层上的液体的分子量的增加而降低。近的研究发现，接触角滞后与表面的分子流动与组装方式、液体渗透和表面膨胀相关。具体到测试接触角滞后的目前有效的工具而言，前进、后退角是的方法。通常，前进角、后退角的测试方法有三种：
(1)增加和减少在固体表面的液体量，形成液－气界面与固体界面的移动。
- When a liquid drop is formed by injecting the liquid from a needle connected to a syringe onto a substrate surface, it is allowed to advance on the fresh solid surface and the measured angle is said to represent the advancing contact angle, θa. For each drop–solid system there is a maximum value of θa before the three-phase line is broken (it should be noted that the stainless steel needle must be kept in the middle of the drop during measurement of θa, on the metal needle surface; alternatively, plastic needles such as Teflon and polypropylene may be used with water). Sometimes, we use increasing of length of contact line to estimate forming of advancing contact angle.
contact angle measurement
- The receding contact angle, θr, can be measured when a previously formed sessile drop on the substrate surface is contracted by applying a suction of the drop liquid through the needle. Precise measurement of θr is very difficult.
These contact angles fall within a range where the advancing contact angles approach a maximum value and receding angles approach a minimum value (θa>θr). Alternately, both advanced and receded angles are measured when the stage on which the solid is held is tilted to the point of incipient motion of the drop.
Both θa and θr depend on the surface roughness (detailed shapes and configurations of the patches or strips) and also on the surface chemical heterogeneity. The direct determination of θa within ±2° is easy, but it is difficult to reduce the relative error to ±0.5°. This is because the direction of a liquid profile rapidly changes with the distance from the three phase contact point. The difference between θa and θr gives the contact angle hysteresis, H, (H ≡θa −θr), which can be quite large, around 5–20° in conventional measurements (or 20–50° in some exceptional cases).
The problem of this approach is distortion of the drop surface caused by the needle. If the needle enters the drop at a point very close to the solid, it may obscure the drop profile. It is best to keep the needle at the middle of the drop. If the needle passes through the upper surface of the drop, there will be some capillary rise of the liquid up the needle and distortion of the surface. (However, it has been claimed by some authors that this capillary rise does not perturb the liquid in the region of the contact line with the solid.) Removing the needle from the drop does not help, because that makes it impossible to study hysteresis. Another problem is the fitting method for this situation is usually used tangent method (such as straight line or similarly polynomial equation such as y=a+bx+cx0.5+d/lnx+e/x2) and it may lead to high data error, small reliability due to its simplified fitting method. Spline fitting method and RealdropTM method adopted in CAST®3.0 may enhance the precision but this (enhance-effect) is not very significant. The third problem is the variable rate of introducing the drop liquid through the needle during determination of θa and the variable rate of withdrawal of the liquid during determination of θr. The sessile drop method is not particularly well adapted to quantitative measurement of the dependence of contact angle on the rate of advance or retreat, because a linear rate of change in drop volume does not correspond to a linear rate of motion of the drop front. An appropriate rate is of the order of 0.01–0.10mmmin−1 linear advance or retreat by using a motor-driven syringe. Also, it is best to specify a constant time allowed before measuring the contact angle after the motion stops, e.g. 1–10 sec, to damp the drop oscillations formed in order to obtain more precise data.
(2) Rotating sample stage:
A sessile drop is formed on a plate of solid substrate gripped at one end onto a motorized or manual rotation stage, which can be rotated to the point of incipient motion of the drop. When the plane of the solid surface reaches a critical slope, the drop starts to roll off. The measured angle at the downhill edge of the drop approaches θa, and the angle at the uphill edge approach θr, as shown right. The angles should be measured immediately prior to the drop starting to slide. The roll-off angle, θroll-off, can also be used to derive thermodynamic conclusions; however, this method is not very reliable, because the determination of a clear and sharp drop image at the instance of sliding is difficult, and also it gives inconsistent results with rough substrates which show a strong pinning behavior with the liquid drop, so that no drop sliding occurs even at a tilt angle of θt = 90°. In addition, some researchers cautioned against this method because it yields values of θa and θr that are strongly dependent on the drop size.
contact angle measurement
(3) increase and decrease volume of liquid and with sample made a hole for dosing or sucking liquid:
This approach is preferred for you due to its reliable and precise and algorithm that CAST®3.0 adopts is ADSATM exactly. It is first promoted by Neumann and co-workers. They made a small hole in the flat substrate sample and first deposited a small drop on the substrate through a needle connected to this hole beneath the substrate. The size of the drop is then increased by feeding more liquid to the drop by means of this needle connected to a motorized syringe. This procedure prevents the drop oscillating and also destruction of the axisymmetric. By this means, they controlled the rate of advance or retreat of the symmetrical sessile drop on the substrate, to measure θa and θr precisely. They also developed a method to determine both the contact angle and surface tension of the liquid by applying a digital image analysis to drop profiles and a computation method named axisymmetric drop shape analysis, ADSA. In this method, an objective function is constructed which expresses the error between the physically observed profile and the theoretical Young-Laplace equation curve; the function is then minimized using an iterative procedure.
The limitation of this method is some solid sample such as glass cannot make a hole easily.
contact angle measuement
