Small But Powerful Optically: Glass Microcapillaries for Studying Complex Fluids or Biological Systems with Submicrolitre Samples under Harsh Conditions

Received: 19 December 2019 Accepted: 28 March 2020 Glass micro-capillaries are the simplest yet most versatile, robust, practical and cheap microfluidic devices. Their small size and high optical quality favour detailed investigation under the optical microscope. Here we first review some of their applications, such as determining contact angles and the observation of tenuous wetting films under harsh conditions of pressure and temperature. We further explore how an optical cusp formed by reflection off the inner wall of a glass capillary may be used to monitor the refractive index of its fluid content. Finally, we illustrate how the above advantages may be put to use in the study of extremophile microorganisms, for example in recreating under the microscope the conditions prevailing on the ocean floors.