Figure 1. (a) Front view of a VFD & (b) schematic diagram of the original VFD19 with recent versions having the spinning guide replaced by a bearing with the other another bearing close to the base of the tube. (b) Reproduced under the terms of the CC BY 3.0 Unported license19. Copyright 2013, Springer Nature.
The VFD has two modes of operation, a confined mode, and a continuous-flow mode, with the former having a finite amount of liquid in a glass tube that is closed at one end and rapidly rotated. The latter has liquids exiting from the top of the tube after being fed continuously into the base or at specific positions along the inside of the tube. Although the capability for processing in continuous flow was the original purpose of the designed technology, processes in the confined mode can also be used for small quantities of liquid, with processing scalability being dependent on the amount of material required. As such, industrial applications with high volumes would require a single large unit20 or multiple units in a parallel array for processing. In contrast, small volume niche applications, for example in medicine, require only a single VFD unit. The VFD itself is relative inexpensive, and is a versatile microfluidic platform for controlling chemical reactivity and selectivity, material synthesis, and probing the structure of self-organized systems, offering a range of benefits over conventional processing19, 15. Micro-mixing and high shear stress are imparted in the dynamic thin film on the surface of the rotating rapidly tube in the VFD. The VFD has been employed successfully for a remarkable diverse range of applications, such as in fabricating nanocarbons21, 22, exfoliating of graphite and boron nitride23, accelerating enzymatic reactions24,25, manipulation of polymer networks26, protein purification27, and indeed protein folding28.
The VFD is particularly useful for regulating the shape and size of NPs for both bottom-up and top-down processing. The rapidly evolving capabilities and innovative processing toolbox for the device allows mapping out synthetic strategies for seemingly endless research and industrial possibilities in the nanomaterials arena alone. Intense micro-mixing and high shear stress in the thin film of liquid in the VFD can be harnessed in a controlled way to discover and improve chemical reactivity beyond what is possible using batch processing where processing is limited by diffusion control. The VFD is proficient in probing the structure of self-organized systems, materials processing, instigating chemical reactions, and rapidly imparting organic modifications to a range of motifs in a controlled fashion. Despite the VFD being unlike typical microfluidics where channels are used with typically laminar fluid flow, it is acquiring distinction as a multipurpose microfluidic platform with novel operating characteristics. Modern-day chemistry focuses on the metrics of incorporating green chemistry into the science at its inception and in this context processing the VFD can be significantly improved for research and industrial applications.
Recent fundamental studies on fluid flow in the VFD have established an understanding that the fluid exhibits resonant behaviors from the constricting boundaries of the glass surface and the meniscus, which define the liquid film thickness. To overcome the inability to directly measure the unique fluid flow in the rotating reference frame in the VFD, materials processing strategies were employed which allowed identifying specific topological mass transport regimes29, namely the spinning top flow normal to the surface of the tube, double-helical flow across the thin film, and spicular or spherical flow, and an area of transition whereby both effects are present. The presence of these topological fluid flows which have specific size and shape was further supported by determining mixing times, temperature profiles, and film thickness for increasing rotational speed. These flow patterns also possess distinctive signatures that make it possible to predict the morphology of nanomaterials processed in the VFD, for instance, in shear-stress induced recrystallization, crystallization, and polymerization, depending on the rotational speeds, thereby providing molds of high-shear topologies, as ‘positive’ and ‘negative’ spicular flow behaviors, which are further detailed below. This is shown in Figure 2 along with “molecularly-drilling” of holes in thin films of preformed polysulfone which correspond to the spatial arrangement of double helical topological fluid flows. We note that optimal processing in the VFD for a myriad of applications is at θ 45° tilt angle and the aforementioned results for this angle uniquely provide the distinct behavioral topological fluid flow regimes, Figure 3. This model introduces a novel idea for creating unique nanomaterials and the organization of matter, now with the ability to have a high level of predictability for the optimal processing in tackling new applications of the VFD.