Department of Chemical and Biomolecular Engineering, NYU Tandon School of Engineering, 6 Metrotech Center, Brooklyn, New York 11201, United States.
J Phys Chem B. 2021 Jul 22;125(28):7828-7839. doi: 10.1021/acs.jpcb.1c02620. Epub 2021 Jul 14.
Tightly focusing a continuous-wave, near-infrared laser beam at the air/solution interface of a millimeter-thick layer of glycine in DO forms a crystal through a polymorphically and spatially controlled nucleation process known as gradient-force laser-induced nucleation or optical-tweezer laser-induced nucleation. However, when this same beam is focused at the glass/solution interface of a film of aqueous glycine, a highly concentrated laser-induced phase-separated (LIPS) solution droplet is formed that does not nucleate while the focusing beam remains on. Two competing theories have emerged about the nature of the LIPS droplet: one proposes that it is a merger of prenucleation metastable nanodroplets and clusters into one large homogeneous "dense liquid droplet", and the other stipulates that it is the result of the partitioning of larger droplets into the new phase, but not a merging of droplets, around the focal point of the beam. In order to determine the nature of the LIPS droplet, dynamic light scattering was used to detect the presence of nanodroplets undergoing Brownian motion within the droplet and to measure their relative size following a range of laser exposure times. The observation of nanodroplets in motion in the center of the LIPS droplet revealed that the application of optical tweezers at the glass/solution interface forms a relatively monodisperse collection of large nanodroplets (>700 nm) concentrated around the focal point of the beam with smaller particles (<100 nm) depleted within the first 2 min of laser exposure. The LIPS droplet quickly reaches a steady state and is not affected by increasing focusing times. These findings allow for a better understanding of the interactions of optical tweezers with aqueous glycine nanodroplets. This understanding will help in studying the fundamental nature of metastable nanodroplets. More practically, laser-induced phase separation makes possible the nucleation-free separation of large nanodroplets from small clusters, facilitating materials technologies such as high purity, polymorphically selective nucleation of crystals and co-crystals used for pharmaceuticals, dyes, and photovoltaics.
连续波近红外激光在 DO 中厚达毫米的甘氨酸层的空气/溶液界面处聚焦，通过一种称为梯度力激光诱导成核或光镊激光诱导成核的多晶态和空间受控成核过程形成晶体。然而，当同一光束聚焦在水合甘氨酸膜的玻璃/溶液界面时，会形成一个高度浓缩的激光诱导相分离（LIPS）溶液液滴，在聚焦光束保持开启时不会成核。关于 LIPS 液滴的性质出现了两种相互竞争的理论：一种理论认为它是预成核亚稳纳米液滴和团簇合并成一个大的均匀“稠密液滴”，另一种理论规定它是较大液滴在光束焦点周围进入新相的结果，而不是液滴的合并。为了确定 LIPS 液滴的性质，使用动态光散射来检测布朗运动中的纳米液滴的存在，并测量它们在一系列激光照射时间后的相对大小。在 LIPS 液滴的中心观察到纳米液滴的运动，表明在玻璃/溶液界面处应用光镊形成了一个相对单分散的大纳米液滴（>700nm）的集合，它们集中在光束的焦点周围，而较小的颗粒（<100nm）在激光照射的前 2 分钟内耗尽。LIPS 液滴迅速达到稳定状态，不受增加聚焦时间的影响。这些发现使我们更好地理解了光镊与水合甘氨酸纳米液滴的相互作用。这种理解将有助于研究亚稳纳米液滴的基本性质。更实际的是，激光诱导相分离使得从较小的团簇中无成核地分离大的纳米液滴成为可能，从而促进了材料技术的发展，如用于制药、染料和光伏的晶体和共晶体的高纯度、多晶态选择性成核。
