Given its origin in kinetic theory and its direct connection to the Boltzmann equation, there has been a persistent effort to use the lattice Boltzmann equation (LBE) for modeling and simulation of nonequilibrium effects in rarefied gases, i.e., the effects due the finite Knudsen number Kn. This presentation will review the experience and knowledge in the quest to use the LBE for rarefied gas dynamics (RGD). We will first provide a summary of the mathematical basis of the LBE, i.e., its direction connection to the Boltzmann equation. The LBE is indeed a finite moment system with low-Mach number expansion of the distribution function on a regular discrete velocity set and a linearized collision model and it is closely related to the discrete velocity models (DVMs) of the Boltzmann equation. For low-Mach-number gaseous flows for which the LBE applies, the linearized Boltzmann equation can be recast as a Fredholm integral equation of the second kind, which can be analytically studied and numerically solved with high precision. By benchmarking against the data obtained from accurate solutions of some canonical flows, e.g., the planar Couette and Poiseuille flows, the LBE is shown to be capable of capturing the Knudsen layer. We hope this presentation can provide an accurate historical perspective, and the current status quo and capabilities of the LBE.

Laser Induced Plasma (LIP) is getting growing interest in recent decades because such plasmas can be produced with great experimental flexibility by simply steering and focusing the laser pulse on the target/sample. For example LIP has been used in analytical chemistry for elemental analysis, as well as in materials science for the production of thin films, material modification, and nanomaterial production and in propulsion applications. Beyond the practical applications, the LIP represents an extremely interesting system from the fundamental point of view, because during the expansion the characteristics and properties of the plasma change dramatically. These changes permit the investigation of atomic plasmas, from non-ideal plasma to ideal plasma in different thermodynamic states. These include Local Thermal Equilibrium (LTE), partial Local Thermal Equilibrium (pLTE) and reactive plasma, with production of molecules as the result of the interaction of the plasma with the surrounding environment. The induction of the plasma laser starts with the excitation of the electrons in the irradiated material and with the generation of free electrons by multi-photon ionization. After a few tens of femtoseconds the ejection of electrons and ions occurs massively. The ablated matter is then heated by the rest of the laser pulse, increasing the kinetic energy of the electrons by inverse bremsstrahlung and promoting further ionization by electron collision and photoionization. Although the control of the ablation process for a given laser pulse is limited, the evolution of the plasma is strictly dependent on the dynamics of the LIP. This implies that it is possible to operate on the plasma characteristics by controlling the expansion features with different background environment. In this work several optical diagnostics, including high temporally and spatially resolved Emission Spectroscopy, Shadowgraphy, fast imaging, laser absorption and scattering have been used for discussing the fundamental aspects of LIP in the frames of different applications.

Fluid mechanics has played a major - albeit sometimes uncredited - role in the global response to the COVID19 pandemic. Well-publicized visualization of exhaled droplet cloud transport helped develop and accelerate acceptance of social distancing guidelines to reduce the airborne transmission of SARS-CoV-2 virus. Less visible but more impactful in ultimately ending the pandemic are the advances in microfluidics coupled with biotechnology that led to development and manufacturing of new COVID19 vaccines at an unprecedented speed and scale. COVID19 vaccines by Pfizer-BioNTech and Moderna are based on messenger RNA (mRNA) encapsulated in lipid nanoparticles (LNP) for delivery of fragile RNA into the cell. The mRNA vaccine production does not require complex and expensive cell culture bioreactors and opens an era when customizable vaccines and therapeutics can be manufactured on demand. In these vaccines, an ionizable lipid forms a shell to preserve and deliver the mRNA to patient cells to prompt production of SARS-CoV-2 spike proteins which triggers and trains the immune system. Microfluidic mixing of lipids, buffers and nucleic acids in an ethanol solution has been the backbone of manufacturing of these vaccines by providing a reliable method of packaging the sensitive molecules in a tight biocompatible particle of about 100 nm. The incredibly complex nature of LNP-mRNA makes them very unstable in the liquid phase requiring cold storage. The cold chain limits the global distribution of vaccines. Efforts by Pfizer and other manufacturers to develop lyophilized formulations of COVID19 vaccines have been reported recently. Lyophilization/freeze-drying requires vacuum (<1 torr) and involves rarefied transport through porous media. We will review the various fluid dynamic and rarefied gas flow aspects of potential breakthrough freeze-drying technologies required for increased global capacity for manufacture of lyophilized pharmaceuticals and vaccines.