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When a bubble rises in a suspension of microscopic particles, the particles that collide with the bubble can stick to it and be brought to the surface, a process used to separate microparticles in a variety of industrial processes ranging from wastewater recovery to mineral flotation. Previous experiments and models for bubble scavenging have focused on bubbles rising in unconfined flow and in narrow microfluidic channels. However, precisely how scavenging is affected by confinement in only a single direction is largely unknown. In this study, experiments and theoretical analysis are carried out for bubble scavenging in a gap, or Hele-Shaw cell, for various gap distances. A novel approach was developed to measure the particles collected by a single bubble by allowing the bubbles to pop and transport these particles to a glass plate where they can be counted. The experiments reveal that the scavenging efficiency increases as the gap narrows up to a factor of four for a gap that is slightly larger than the bubble diameter. To understand these results, a numerical simulation of the flow field around the bubbles for different confinement gaps. An analytical expression is developed for the scavenging efficiency based on a combination of 2D and 3D potential flow, and other factors such as the bubble shape and trajectory are evaluated.

RNA plays an indispensable role in mammalian cell functions. Cas13, a class of RNA-guided ribonuclease, is a flexible tool for modifying and regulating coding and non-coding RNAs, with enormous potential for creating new cell functions. However, the lack of control over Cas13 activity has limited its cell engineering capability. Here, I present the CRISTAL (Control of RNA with Inducible SpliT CAs13 Orthologs and Exogenous Ligands) platform. CRISTAL is powered by a collection (10 total) of orthogonal split inducible Cas13s that can be turned ON or OFF via small molecules in multiple cell types, providing precise temporal control. Also, I engineered Cas13 logic circuits that can respond to endogenous signaling and exogenous small molecule inputs. Furthermore, the orthogonality, low leakiness, and high dynamic range of the inducible Cas13d and Cas13b enable the design and construction of a robust incoherent feedforward loop, leading to near-perfect and tunable adaptation response. Finally, using the inducible Cas13s, I achieve simultaneous multiplexed control of multiple genes in vitro and in mice. Together, the CRISTAL design represents a powerful platform for precisely regulating RNA dynamics to advance cell engineering and elucidate RNA biology.

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