Milani Matteo, Phou Ty, Ligoure Christian, Cipelletti Luca, Ramos Laurence
Laboratoire Charles Coulomb (L2C), Université Montpellier, CNRS, Montpellier, France.
Soft Matter. 2023 Sep 20;19(36):6968-6977. doi: 10.1039/d3sm00625e.
The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops sitting on a hydrophobic surface, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities, invagination and cracking. While permanent aggregation of nanoparticles accompanies the second instability, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops sitting on a hydrophobic surface.
胶体悬浮液液滴的蒸发在许多情况下都起着重要作用，例如奶粉的生产、功能性超粒子的合成，以及病毒和细菌在气溶胶或表面液滴中的存活。蒸发液滴中胶体颗粒的存在最终会导致形成一个致密的壳层，该壳层可能会发生形状不稳定性。先前的研究表明，对于蒸发非常快的液滴，当颗粒形成由随机紧密堆积的永久聚集颗粒构成的刚性多孔固体时，就会发生不稳定性。然而，迄今为止，尚无测量方法能够直接验证这种情况，也无法评估其是否也适用于较低蒸发速率下干燥的液滴，这严重限制了我们对这一现象的理解以及在应用中利用它的可能性。在这里，我们结合宏观成像以及对位于疏水表面上的干燥液滴中胶体纳米颗粒微观动力学的空间和时间分辨测量，测量壳层厚度的演变以及纳米颗粒的空间分布和迁移率。我们发现，在高于阈值蒸发速率时，液滴会依次经历两种不同的形状不稳定性，即内陷和破裂。虽然纳米颗粒的永久聚集伴随着第二种不稳定性，正如先前关于快速蒸发液滴的研究所假设的那样，但我们表明第一种不稳定性是由壳层的可逆玻璃化转变引起的，这是迄今为止尚未报道过的。我们对我们的发现进行了合理化分析，并在疏水表面上胶体液滴干燥的统一状态图框架内讨论了它们的意义。
