A cavitation, or the production and nucleation of reactive vapor bubbles within a liquid, is a fundamental phenomenon in fluid dynamics with wide-ranging applications from industrial-scale operations, food engineering and biomedical science.
The study of cavitation’s in isotropic liquids has been well studied, but is currently lacking for anisotropic liquids. A team of international Researchers have now used microfluidic and computational approaches to determine the cavitation dynamics of a Stokes flow using an anisotropic liquid.
Cavitation’s are one of the oldest and most researched phenomena in fluid dynamics and are normally induced through localized heating of a fluid above its boiling temperature, or through the decrease in a fluid’s pressure to below its saturated vapor pressure.
Cavitation’s are high energy voids which generally occur when a liquid is subject to tensile stresses. At a critical point, known as the breaking tension or the cavitation threshold, the liquid will experience a negative pressure and the interface will break down (i.e. implode) creating a void- also known as a cavitation. Cavitation’s can be produced using a range of techniques and equipment, with sonochemical probes being a very common choice.
Of all the cavitation types, there is one known as a hydrodynamic cavitation. These cavitation’s occur under a hydrodynamic flow but have not been widely studied until now, due to issues with multiparameter dependencies arising from a multitude of composition and systematic factors.
Anisotropic liquids are a special kind of complex, non-Newtonian liquid where the molecules exhibit a long-range order with respect to their orientation and/or their position. Common examples in chemistry of anisotropic liquids are liquid crystals and polymeric liquids.
Until now, a study towards the cavitation’s in anisotropic liquids has not been undertaken, despite its importance in the biomedical fields where lithotripsy methods can cause intraluminal bubble expansion and capillary ruptures in a person’s blood vessels.
The Researchers used microfluidic experiments and nonequilibrium molecular dynamics (MD) simulations to study and predict the hydrodynamic cavitation’s in a nematic phase liquid crystal at room temperature, and to capture the inception and growth of the cavitation.
The cavitation’s in the microfluidic device occurred due to the nematic 5CB phase flowing past a cylindrical pillar placed within a linear microfluidic channel. Upon the pillar, the Researchers pinned a vapor-liquid interface which encased the cavitation domain. The nucleation of the cavitation’s in these domains was found to be due to a drop-in pressure of the flow around the cylinders.
The microfluidic devices were created using polydimethylsiloxane (PDMS), alongside oxygen plasma treatments and soft lithography casting. The channels were composed of cylindrical tubes of varying diameters and were connected to outer tubes composed of Teflon.
The microfluidic channels pumped the fluid at controlled rates using a gear pump (neMESYS, Cetoni) and a gas-tight microlitre syringe (1001LT, Hamilton Bonaduz). The Researchers also used a Nikon Eclipse LV100 polarizing microscope to identify the cavitation domain and the orientation of the flowing liquid crystal molecules.
The inception and growth of the cavitation domain was found to take place in the Stokes regime, that is when the Reynolds number (the number when a laminar flow becomes turbulent) is less than 1. The Researchers also found no cavitation’s in isotropic liquids flowing under similar hydrodynamic parameters.
The Researchers also identified a large range of Ericksen numbers, i.e. numbers which characterize the nematofludic conditions in the system, where the cavitation’s are reproducible, stable and sustainable. The Researchers found a range between 200 and 500.
In the Ericksen number range, the Researchers found that the cavitation domain grew until it reached a saturation volume. Whereas above these numbers, i.e. greater than 500, the domain became unstable and shrunk in size. In all the experiments, the Researchers found that the cavitation volume was localized around the stagnation point downstream of the pillars.
Using MD calculations, the Researchers also found that the critical Reynolds number (the number for cavitation’s to occur) was inversely proportional the order parameter of the anisotropic liquid. This is different to isotropic flows, which have virtually no order parameter, and the critical Reynolds number was found to be up to 50% lower in anisotropic fluids compared to their isotropic counterparts.
The study is a big step into understanding the effect of cavitation in anisotropic liquids and compliments the extensive studies on isotropic cavitation’s. Aside from garnering knowledge, it also opens doors into the production of a novel control parameter based around cavitation’s at low Reynold number flows. It also has the potential to assist with biomedical research which utilizes one of the most common anisotropic liquids- blood.
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“Hydrodynamic cavitation in Stokes flow of anisotropic fluids”- Stieger et al, Nature Communications, 2017, DOI: 10.1038/ncomms15550