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.