Measurement of Liquid Transport in a Single Diatom Frustule via In-Situ Optical Microscopy

Diatoms are a type of microscopic phytoplankton that exist in various aquatic environments (oceans, lakes, rivers). They are unicellular organisms enclosed in intricate, silica-rich microshells known as frustules. Notably, diatom frustules can possess multilevel hierarchical 3-D pore patterns with the smallest pore sizes down to a few tens of nanometers. Such unique 3-D multiscale features cannot be easily achieved with state-of-the-art cleanroom fabrication techniques. For this reason, diatom frustules have attracted attention in a wide range of applications, from nanotechnology to biotechnology (e.g., for drug delivery, wastewater treatment, biomolecule, and gas sensors). To support these crucial applications, it is essential to possess a comprehensive understanding of the fundamental principles governing liquid transport within diatom frustules. However, characterizing liquid transport through the hierarchical pores in a single microscopic diatom frustule is fundamentally challenging. To overcome these challenges, we developed a microfluidic test rig equipped with high-resolution optical microscopy. This characterization apparatus is capable of detecting liquid propagation from multiple angles with high spatial resolution and with a high frame capture rate. We have used this apparatus to identify microscopic features affecting liquid transport in <i>Coscinodiscus wailesii</i> diatom frustules and, for the first time, obtained key liquid transport characteristics of such frustules (including capillarity, wickability, and permeability). The optical metrology-based test rig developed in this work can be a useful platform for evaluating microfluidic behavior in various complex structures. This study is based on work supported by the Air Force Office of Scientific Research under Award Number FA9550-23-1-0055.Diatoms are a type of microscopic phytoplankton that exist in various aquatic environments (oceans, lakes, rivers). They are unicellular organisms enclosed in intricate, silica-rich microshells known as frustules. Notably, diatom frustules can possess multilevel hierarchical 3-D pore patterns with the smallest pore sizes down to a few tens of nanometers. Such unique 3-D multiscale features cannot be easily achieved with state-of-the-art cleanroom fabrication techniques. For this reason, diatom frustules have attracted attention in a wide range of applications, from nanotechnology to biotechnology (e.g., for drug delivery, wastewater treatment, biomolecule, and gas sensors). To support these crucial applications, it is essential to possess a comprehensive understanding of the fundamental principles governing liquid transport within diatom frustules. However, characterizing liquid transport through the hierarchical pores in a single microscopic diatom frustule is fundamentally challenging. To overcome these challenges, we developed a microfluidic test rig equipped with high-resolution optical microscopy. This characterization apparatus is capable of detecting liquid propagation from multiple angles with high spatial resolution and with a high frame capture rate. We have used this apparatus to identify microscopic features affecting liquid transport in <i>Coscinodiscus wailesii</i> diatom frustules and, for the first time, obtained key liquid transport characteristics of such frustules (including capillarity, wickability, and permeability). The optical metrology-based test rig developed in this work can be a useful platform for evaluating microfluidic behavior in various complex structures. This study is based on work supported by the Air Force Office of Scientific Research under Award Number FA9550-23-1-0055.