Maestro Armando, Rio Emmanuelle, Drenckhan Wiebke, Langevin Dominique, Salonen Anniina
Laboratoire de Physique des Solides, Bâtiment 510, Université Paris-Sud XI, 91405-Orsay, France.
Soft Matter. 2014 Sep 28;10(36):6975-83. doi: 10.1039/c4sm00047a.
We have studied foams stabilised by surfactant-decorated nanoparticles adsorbed at the bubble surfaces. We show that the controlled compression of a single bubble allows one to understand the coarsening behavior of these foams. When bubbles are compressed, the particles become tightly packed in the surface layer. They lose their mobility, and the interface becomes solid-like when the jammed state is reached. Further compression leads to interfacial buckling characterised by crumpled surfaces. We find that the surface concentration of particles at which the jamming and the buckling transitions occur are independent of the surfactant concentration. This is a surprising feature. It suggests that the surfactants are mandatory to help the particles adsorb at the interface and that they change the equilibrium surface concentration of the decorated particles. But they do not affect the surface properties once the particles are adsorbed. We measured the compression elastic modulus of the surface in the jammed state and found it to be compatible with the Gibbs condition for which the spontaneous dissolution of bubbles is arrested. Due to this effect, the coarsening process of a foam composed of many close-packed bubbles occurs in two steps. In the first step, coarsening is slow and coalescence of the bigger bubbles is observed. In the second step, a number of very small bubbles remains, which exhibit crumpled surfaces and are stable over long times. This suggests that foam coarsening is arrested once the smallest bubbles become fully covered after the initial shrinking step.
我们研究了由吸附在气泡表面的表面活性剂修饰纳米颗粒稳定的泡沫。我们表明，对单个气泡进行可控压缩能够让人理解这些泡沫的粗化行为。当气泡被压缩时，颗粒在表面层紧密堆积。它们失去流动性，当达到堵塞状态时，界面变得像固体一样。进一步压缩会导致以表面起皱为特征的界面屈曲。我们发现发生堵塞和屈曲转变时颗粒的表面浓度与表面活性剂浓度无关。这是一个令人惊讶的特征。这表明表面活性剂对于帮助颗粒吸附在界面上是必不可少的，并且它们会改变修饰颗粒的平衡表面浓度。但是一旦颗粒被吸附，它们不会影响表面性质。我们测量了堵塞状态下表面的压缩弹性模量，发现它与阻止气泡自发溶解的吉布斯条件相符。由于这种效应，由许多紧密堆积气泡组成的泡沫的粗化过程分两步进行。在第一步中，粗化缓慢，观察到较大气泡的合并。在第二步中，会留下一些非常小的气泡，它们的表面起皱且在很长时间内保持稳定。这表明一旦最小的气泡在初始收缩步骤后被完全覆盖，泡沫粗化就会停止。
