The structural and chemical uniformity of traditional emulsions facilitates their production; however, this simplicity also limits the scope of their applications. Multiphase or ‘multiple’ emulsions have increased complexity and potentially broader utility than conventional emulsions, owing to their internalized phases, enabling chemical compartmentalization, control of active-ingredient release and complex particle templates. However, multiple-emulsion droplets prepared with conventional methods are too large for emerging applications. As a result, there has been increased research interest in the development of multiple nanoemulsions, whereby the nanoscale droplet size overcomes many of the limitations of more common, micrometre-scale multiple emulsions. Although numerous successful demonstrations of multiple-nanoemulsion production have emerged, there remain considerable challenges to improve stability, control over internal structure and characterization of multiphase nanodroplets. Overcoming these challenges provides unique opportunities to exploit the hierarchical structure, enhanced colloidal stability and multiphase chemical solubilization for applications, such as delivery vehicles, chemical sinks and particle templates, with functions that are otherwise inaccessible. In this Review, we summarize the techniques used to generate and characterize multiple nanoemulsions, theories to understand their formation and stability, and discuss current and future applications that are, or may be, enabled by their unique structures and chemistries.

传统乳液的结构和化学均匀性有助于其生产；但是，这种简单性也限制了它们的应用范围。由于其内在的相，多相或“多”乳液比传统乳液具有更高的复杂性并可能具有更广泛的用途，可实现化学分隔，控制活性成分释放和复杂颗粒模板。然而，对于常规应用而言，用常规方法制备的多乳液液滴太大。结果，对多种纳米乳剂的开发有了越来越多的研究兴趣，其中纳米级液滴尺寸克服了更常见的微米级多种乳剂的许多限制。尽管出现了许多成功的多纳米乳液生产的示范，在提高稳定性，控制内部结构和表征多相纳米液滴方面仍然存在着巨大的挑战。克服这些挑战提供了独特的机会，可以利用其层次结构，增强的胶体稳定性和多相化学增溶性来实现应用，例如运输工具，化学接收器和颗粒模板等，而这些功能是其他功能无法实现的。在本综述中，我们总结了用于生成和表征多种纳米乳剂的技术，了解其形成和稳定性的理论，并讨论了其独特的结构和化学性质可能或可能引起的当前和将来的应用。克服这些挑战提供了独特的机会，可以利用其层次结构，增强的胶体稳定性和多相化学增溶性来实现应用，例如运输工具，化学接收器和颗粒模板等，而这些功能是其他功能无法实现的。在本综述中，我们总结了用于生成和表征多种纳米乳剂的技术，了解其形成和稳定性的理论，并讨论了其独特的结构和化学性质可能或可能引起的当前和将来的应用。克服这些挑战提供了独特的机会，可以利用其层次结构，增强的胶体稳定性和多相化学增溶性来实现应用，例如运输工具，化学接收器和颗粒模板等，而这些功能是其他功能无法实现的。在本综述中，我们总结了用于生成和表征多种纳米乳剂的技术，了解其形成和稳定性的理论，并讨论了其独特的结构和化学性质可能或可能引起的当前和将来的应用。