High Viscosity Extruders
High Viscosity Extruders (HVI) are hydraulic micro-jetting devices designed to stream viscous, toothpaste-like materials.
High viscosity material is loaded into the reservoir and pushed by a plunger (polytetrafluoroethylene ball) through the short capillary (with 50-100 µm inner diameter). The polytetrafluoroethylene (PTFE) ball is driven here by a piston which additionally works as a hydraulic pressure amplifier. The ratio of cross-sections of the hydraulic stage bore and the sample reservoir bore will define the pressure amplification. The jet diameter in this case is approximately equal to the capillary inner diameter.
HVI devices usually use a sheath gas blowing through the specifically shaped gas aperture to keep the extruded jet stable and straight. The flow rate for HVI is typically 0.01-3 µL/min and jet speed 1-10 mm/s.
Fig.1. High Viscosity Injector (HVI). The hydraulic stage is pressurized by an HPLC pump. The piston pressurizes the sample in the reservoir and drives it through the capillary. The ceramic nozzle ensures that sheath gas surrounds the extruded high viscosity material and keeps the flow on the axis. Weierstall et al.
Different types of high viscosity media can be used to produce high viscosity jets. The most common media are Lipid Cubic Phase (LCP), poly-ethylene-oxide (PEO), Cellulose, and SuperLube. The choice of media depends on its stability as well as the stability of the crystal suspended in it. The sample consumption is significantly reduced for HVI compared to other liquid jet injection methods.
However, the significant jet thickness might reduce the noise-to-signal ratio. Additionally, high viscosity jets might produce strong diffraction rings which can mask low-resolution Bragg reflections.
Pros and Cons of High Viscosity Extruders