Physics of Complex Fluids and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
Langmuir. 2010 Aug 17;26(16):13245-50. doi: 10.1021/la102120h.
We present temperature-dependent atomic force microscope (AFM) measurements in force-distance mode of confined 1-dodecanol. Upon approach of the AFM-tip toward the highly oriented pyrolytic graphite (HOPG) surface, the final liquid film--only a few nanometers thin--is squeezed out in discrete layers. We find that both the force needed to squeeze out these layers and the number of structured layers strongly increase as the freezing temperature is approached. Surprisingly the force increases nonmonotonically and show a local maximum around 3 degrees and a local minimum at 1 degree above the bulk melting point of the liquid. We attribute this result to changes in epitaxial effects between 1-dodecanol and the HOPG surface close to the melting point of the liquid. To test this hypothesis we performed the same measurements in hexadecane, a similar carbon-chain molecule, and octamethylcyclotetrasiloxane (OMCTS), a quasi-spherical molecule. Hexadecane shows the same maximum in the squeeze-out force at 4-5 degrees and a minimum at 1-2 degrees above the freezing temperature of the liquid, while the squeeze-out of OMCTS was found to be independent of temperature.
我们呈现了在原子力显微镜（AFM）力距模式下，对受限的 1-十二醇进行的温度依赖的原子力显微镜（AFM）测量。当 AFM 针尖接近高定向热解石墨（HOPG）表面时，最终的液体薄膜——只有几纳米厚——被挤出成离散的层。我们发现，挤出这些层所需的力以及结构化层的数量都随着冷冻温度的接近而强烈增加。令人惊讶的是，力的增加是非单调的，在液体的体相熔点以上 1 度左右呈现局部最大值，在 3 度左右呈现局部最小值。我们将这一结果归因于接近液体熔点时 1-十二醇和 HOPG 表面之间的外延效应的变化。为了验证这一假设，我们在十六烷（一种类似的碳链分子）和八甲基环四硅氧烷（OMCTS）（一种准球形分子）中进行了相同的测量。十六烷在液体的冷冻温度以上 4-5 度处表现出相同的最大挤出力和 1-2 度处的最小挤出力，而 OMCTS 的挤出则被发现与温度无关。
