平衡态千兆赫声学揭示液体中的长程限域效应
Equilibrium gigahertz acoustics reveals long-range confinement in liquids
AI总结：
本研究开发基于超快激光超声的非侵入式全光学千兆赫声学技术，揭示纳米级限域液体的长程类固体等特殊力学行为，为液体限域探测提供新方法。
AI中文摘要：
理解纳米级间隙中限域液体的力学性质与体相行为的差异，对于生物物理学、润滑、催化、电化学和表面科学至关重要。然而，表征超薄限域液体仍极具挑战，因为现有诸多方法依赖于破坏性或侵入式的接触型技术，大多仅能测量低频范围内的液体流动。本研究提出一种基于超快激光超声的非侵入式全光学技术，可在千兆赫频率范围探测平衡态下的限域液体。该方法测量经液体层传输的时域布里渊散射信号的相位与振幅，液体层厚度以亚纳米级有效采样逐步变化。结合声学传播与光学探测的数值建模，这些信号可用于提取限域液体随厚度变化的声速与衰减。研究表明，纳米级限域会改变甘油、液晶8CB及丁基类离子液体的千兆赫声学响应，且这种效应发生在远长于预期的空间尺度上，覆盖从几纳米到几十纳米的范围，揭示了界面束缚层、声学硬化以及限域下增强的类固体行为。本研究结果开辟了一条探测液体限域的新路径：以千兆赫频率动态测量，同时扰动足够微弱以维持限域液体的平衡态。
英文摘要：
Understanding how the mechanical properties of liquids confined within nanometer-scale gaps differ from bulk behavior is central to biophysics, lubrication, catalysis, electrochemistry, and surface science. Yet the characterization of ultrathin confined liquids remains challenging, as many existing approaches rely on destructive or intrusive contact-based techniques, mostly measuring the liquid flow in the low frequency regime. Here, we present a non-invasive, all-optical technique based on ultrafast laser ultrasonics that probes confined liquids at equilibrium in the gigahertz frequency range. The method measures the phase and amplitude of time-domain Brillouin scattering signals transmitted through liquid layers whose thickness is varied step by step with subnanometer effective sampling. Supported by numerical modeling of acoustic propagation and optical detection, these signals allow us to extract the thickness-dependent acoustic velocity and attenuation of confined liquids. We show that nanometric confinement modifies the GHz acoustic response of glycerol, the liquid crystal 8CB, and a butyl-based ionic liquid over unexpectedly long spatial scales. These effects extend from a few nanometers to several tens of nanometers and reveal bound interfacial layers, acoustic stiffening, and enhanced solid-like behavior under confinement. Our results open a route to probing liquid confinement in a scarcely explored regime: dynamically measured at gigahertz frequencies, yet sufficiently weakly perturbative to preserve the equilibrium confined state.
