Thickness-Dependent Phase Transition Drives Nanoridge-to-Mesa Instability in Micellar Freestanding Films.
Thickness-Dependent Phase Transition Drives Nanoridge-to-Mesa Instability in Micellar Freestanding Films.
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DOI：
10.1021/acs.langmuir.8b01010
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发表时间：
2018-06
期刊：
影响因子：
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通讯作者：
Yiran Zhang;Vivek Sharma
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作者：
Yiran Zhang;Vivek Sharma
了解通量和不稳定性内独立的泡沫膜是必要的，以更好地理解和控制，泡沫和乳液的稳定性和寿命。在胶束泡沫膜中，限制诱导的胶束分层导致逐步变薄或分层，其通过较薄、较暗的域的膨胀而发生。通常，由于纳米脊到梅萨的不稳定性，一个或多个较亮的白色斑点或“台面”出现在较薄域和较厚（较暗）周围膜之间的圆形移动前沿处。先前的研究假设不稳定性和白色斑点的出现类似于毛细管驱动的瑞利不稳定性，导致分裂的相干液体射流。使用我们最近开发的IDIOM（干涉数字成像光学显微镜）协议，我们以精致的时空分辨率（厚度<1 nm，时间<1 ms）表征了纳米脊到梅萨的不稳定性。这种不稳定性可以被归类为瑞利不稳定性，如果一个类似的序列的厚和薄的波动是可视化周围的扩展域。然而，定量分析表明，只有台面的大小增长后的不稳定性，而其余的纳米脊保持其形状。通过类比到成分不同的区域的相分离，我们表明，在分层膜中的厚台面的自发成核是由自由能泛函的振荡性质驱动的相变。
Understanding fluxes and instabilities within freestanding ultrathin films is necessary for a better understanding of, and control over, the stability and lifetime of foams and emulsions. In micellar foam films, confinement-induced layering of micelles leads to stepwise thinning or stratification that occurs by the expansion of thinner, darker domains. Often, because of a nanoridge-to-mesa instability, one or more brighter white spots or "mesas" appear at the circular moving front between thinner domains and the thicker (less dark) surrounding film. Previous studies assume that the instability and the appearance of white spots are similar to the capillarity-driven Rayleigh instability that leads to the breakup of a coherent liquid jet. Using the IDIOM (interferometry digital imaging optical microscopy) protocols we recently developed, we characterize the nanoridge-to-mesa instability with exquisite spatiotemporal resolution (thickness <1 nm, time <1 ms). The instability could be classified as a Rayleigh instability if a similar sequence of thick and thin undulations is visualized around the expanding domains. However, quantitative analysis reveals that only mesas grow in size after the instability, whereas the rest of the nanoridge preserves its shape. By analogy to the phase separation into compositionally distinct regions, we show that the spontaneous nucleation of thicker mesas in stratifying films is a phase transition driven by the oscillatory nature of the free-energy functional.