自由面射流作为流体力学领域的重要研究问题，其动力学行为与调控机理在自然界与工业应用中具有广泛的研究价值。随着近年来激光微加工技术和无创医疗设备的快速发展，自由面射流研究逐渐从基础理论向高精度应用场景延伸，应需而生的激光诱导转移（LIT，laser-induced transfer）技术和无针注射（needle-free injection）技术在基础理论和工程应用两个层面均取得突破性进展，有关于自由面射流的产生机制与调控规律的研究呈现出更显著的学科交叉特征，针对其的可控生成与精准调控方法研究呈现出更显著的应用价值。

在技术应用层面，激光诱导转移技术通过空化气泡快速膨胀诱导自由面形成微尺度射流，经由射流产生的液滴实现物质的转移；无针注射技术则通过容器中弯月面附近的流动聚焦产生高速射流，实现无创透皮给药。然而，现有技术仍面临关键科学问题：激光诱导转移过程中次级水冠产生的卫星液滴会引发非预期的物质沉积，而无针注射技术中容器依赖性的凹界面形成机制限制了系统可扩展性。针对上述问题，本研究采用实验研究与数值模拟相结合的方式，重点研究了激光诱导转移中的次级水冠生成机制与控制问题，设计形成了无针注射中不依赖容器的聚焦射流生成方法，以期为相关技术的应用提供有益见解。

本文的具体研究包括如下三个方面：

（1）次级水冠生成机制研究。

本文基于开源平台OpenFOAM中的可压缩几何VOF求解器，针对单个空化气泡与自由面相互作用过程进行了全面的分析，根据自由面的运动特征与几何变化，将次级水冠的生成过程划分为凹界面形成、水冠生成以及水冠发展三个阶段。在各个阶段中，通过联系气泡行为引发的流场变化与自由面行为变化，阐述了次级水冠的生成机制，即气泡收缩时引发的流场压强激增、凹界面处产生的流动聚焦以及气泡再膨胀所引发的动量传递这三者的综合影响。

（2）基于串列双空泡的次级水冠控制研究。

阐明次级水冠生成机制的基础上，本文在近自由面空化气泡的下方引入新的空化气泡，形成串列双空泡构型，通过两空化气泡的相互作用，实现了对次级水冠的控制。通过与单空泡工况中的次级水冠速度进行对比，双空泡工况中的次级水冠的控制状态可分为促进型和抑制型。次级水冠的控制与空泡融合有着明显关联。由于两空泡发生融合，导致收缩过程周期增长，流场的压强升高较为缓慢，因而相较于单空泡工况，双空泡工况中界面上的压力梯度与流动聚焦效应受到削弱。但由于新空泡的引入，融合气泡再膨胀对自由面产生的动量传递要强于单空泡工况。进而，水冠的促进或抑制，取决于气泡再膨胀所增强的动量传递作用能否弥补融合气泡收缩过程中的削弱作用。在此基础上，本文对次级水冠进行了参数化研究，形成了反映次级水冠状态的控制相图。

（3）基于开放空腔与空化气泡相互作用的聚焦射流生成方法。

本文通过在极接近自由面处引入快速膨胀的空化气泡，形成一个半球形的开放空腔，进而实现不依赖容器形成一个凹界面。在空腔下方一定距离内引入另一个空化气泡，通过与空腔的相互作用形成射流。根据空腔和气泡是否相连，本文将射流划分为不稳定射流和稳定射流两大类，其中稳定射流即为无针注射技术中提出的聚焦射流。对射流机理的分析源于空腔伸长和射流形成两个过程。在空腔伸长过程中，空腔向下迁移至最大速度后迅速减速，直至达到最大伸长量并在其下方产生滞止压强；在射流形成过程中，空腔底部的凹界面在滞止压强的驱动下，由于几何效应发生流动聚焦，进而产生聚焦射流。基于对现象和机理的诠释，本文提出了一个理论模型，以获得滞止压强并推导出射流速度。最终得到了空腔底部最大伸长量l_m与射流速度U_jet的线性标度律，并进行了数值验证。

Free surface jets, as a pivotal research topic in fluid mechanics, exhibit extensive research value in both natural phenomena and industrial applications due to their dynamic behaviors and control mechanisms. With recent advancements in laser micromachining and noninvasive medical technologies, research on free surface jets has progressively extended from fundamental theories to high-precision applications. Emerging technologies such as laser-induced transfer (LIT) and needle-free injection have achieved breakthroughs in theoretical frameworks and engineering implementations. These developments highlight the increasingly interdisciplinary nature of studies on jet generation mechanisms and control strategies, while emphasizing the growing practical significance of precise jet manipulation.

At the application level, LIT technology utilizes rapid cavitation bubble expansion to induce micro-scale jets from free surfaces, enabling material transfer through jet-derived droplets. Conversely, needle-free injection generates high-speed jets via flow focusing near meniscus interfaces in containers for transdermal drug delivery. However, critical challenges persist: Satellite droplets from secondary crowns in LIT cause unintended material deposition, while container-dependent concave interface formation in needle-free injection limits system scalability. To address these issues, this study combines experimental investigations with numerical simulations to systematically explore the generation mechanism and control strategies of secondary crowns in LIT, while proposing a container-independent focused jet generation method for needle-free injection. The findings provide valuable insights for advancing these cutting-edge technologies.

The present study encompasses three principal research components:

(1) Mechanism of Secondary Crown Formation.

Employing the compressible geometric VOF solver in OpenFOAM, this work comprehensively analyzes the interaction between a single cavitation bubble and a free surface. The secondary crown formation is categorized into three distinct stages based on free surface kinematics and geometric evolution: (a) concave interface formation, (b) crown formation, and (c) crown development. The generation mechanism of secondary crown is attributed to the synergistic effects of pressure distortion induced by bubble collapse, flow focusing at concave interfaces, and momentum transfer induced by bubble re-expansion.

(2) Crown Control from Tandem Bubble Pair.

Building upon the elucidated mechanism of secondary crown formation, this study introduces another cavitation bubble beneath the primary one to establish a tandem bubble pair configuration. The interaction between the two bubbles enables effective control of secondary crowns. Comparative analysis with single-bubble cases reveals two distinct control modes: enhancement and inhibition of crown dynamics. Bubble coalescence plays a critical role, prolonging the collapse duration and moderating pressure rise in the flow field. This results in attenuated pressure gradients and weakened flow focusing effects at the interface compared to single-bubble cases. However, the coalescent bubble's intensified momentum transfer during re-expansion surpasses that of single bubbles. The transition between enhancement and suppression modes depends on whether the amplified momentum transfer compensates for the diminished effects during coalescent bubble's collapse. On this basis, a parametric investigation of secondary crown was conducted and a control phase diagram reflecting the state of the secondary crown was formed.

(3) Focused Jet Production via Open Cavity-Bubble Interaction.

By initiating a rapidly expanding cavitation bubble in proximity to the free surface, a hemispherical open cavity is created, forming a concave interface without external confinement. Another bubble is introduced at a controlled distance below the cavity, with their synergistic interaction inducing jet formation. The jets are classified into two regimes based on cavity-bubble connectivity: unstable jets (disconnected interaction) and stable focused jets (connected interaction), the latter aligning with needle-free injection requirements. The analysis of the jet mechanism stems from the two processes of cavity elongation and jet formation. During cavity elongation, the cavity accelerates downward until reaching maximum velocity, followed by abrupt deceleration that generates stagnation pressure at its maximum elongation. During the jet formation, the concave interface at the bottom of the cavity is driven by the stagnation pressure, and flow focusing occurs due to geometrical effects, which in turn generates a focused jet. A theoretical model was established to quantify stagnation pressure and predict jet velocity, yielding a linear scaling law between maximum cavity elongation (l_m) and jet velocity (U_jet) which was verified numerically.