(618bn) Bio-Inspired Hierarchical Vascular Networks: Electrohydrodynamic Viscous Fingering

Vascular networks provide a
method to distribute fluid throughout a system. Artificial vascular materials
with enhanced properties are being developed that could ultimately be
integrated into systems reliant upon fluid transport while retaining their
structural properties. An uninterrupted and controllable supply of liquid is
optimal for many applications such as continual self-healing materials, in-situ
delivery of optically index matched fluids, thermal management (sweating)
and drug delivery systems could benefit from a bio-inspired vascular approach
that combines complex network geometries with minimal processing parameters. One
such approach to induce vascular networks whilst mimicking nature's design is
electrohydrodynamic viscous fingering (EHVF).

(a)   
                                                    (b)
(c)

Figure 1. Optical images of EHVF (a) in a 1,000 cSt silicone
oil system containing ~ 60 v/v% glass beads, (b) interfacial polymerization of of
hexamethylene diamine and sodium chloride in water and sebocyl chloride in
10,000 cSt silicon oil, and in three dimensions (3DEHVF) using fumed silica in
an index matched fluid.

Viscous fingering (VF) is a
phenomenon that occurs when a low viscosity liquid is forced through a high
viscosity fluid or matrix. The flowing liquid will branch, or form fingers due
to capillary and viscous forces in the high viscosity material. EHVF is a
modification on viscous fingering in which a DC voltage is applied to the low
viscosity conductive fluid (Fig. 1a) and forced through a dielectric
matrix material. The application of a large electrical potential, 10-60 kV, induces
fingers with a reduction in size and an increased branching behavior. The
ensuing patterns mimic those found in biology and geology (lung tissue and
plants as well as river beds). Observation of VF and EHVF requires Hele-Shaw
conditions in which a 2D system must possess a thin gap (0.8 mm used in these
experiments) or in a porous system. In the 2D instance a silicone oil system is
used as the matrix material, the surfactant concentration was optimized, through
a reduction in the interfacial tension, thereby producing a branched pattern of
small diameter fingers while still maintaining continuity when dyed water is
pushed through the system. Various loadings of glass beads where subsequently
used to represent a more 3D system. Typically, in a two fluid system, the
fingers relax as soon as the applied voltage is removed. Addition of glass
beads, up to 60 v/v%, aids in a retardation of finger relaxation while
producing fine channels throughout the porous system. Delayed relaxation allows
for greater control of the curing process in UV-curable systems, such as
polydimethylsiloxane (PDMS). Fabrics, woven and random glass fiber mats were
also investigated as a matrix to provide a different porosity to the system. Matrix
filling was used as a method to reduce finger relaxation and allow for curing
to occur after the voltage was turned off. Interfacial polymerization EHVF
(IPEHVF), a technique in which the polymerization occurs at the interface of
two materials, was also studied in producing fingers in the silicone oil phase.
For EHVF to be studied in a similar system, hexamethylene diamine (C6H16N2) and
sodium chloride were dissolved in the water phase, while sebacoyl chloride
(C10H16Cl2O2) was disperse in the silicone oil phase. Robust, polymerized,
fingers were formed (Fig. 1b) and remained after the voltage was
turned off. These fingers were subsequently filled with water to show fluid
transport. In moving toward a true 3D system, materials such as fumed silica (Fig
1c) and crushed glass were investigated under 3D (porous) Hele-Shaw
conditions.