(568h) Techniques for Hierarchical Bio-Inspired Vascular Networks: Electrohydrodynamic Viscous Fingering and Electrical Treeing

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. Two
such approaches to induce vascular networks whilst mimicking nature's design are
electrohydrodynamic viscous fingering (EHVF) and electrical treeing (ET).

(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.

                (a)
     (b)                                         (c)
(d)

Figure 2. Optical image of ET (a) in a EPON 828/PACM system,
(b) under AC driven electrical current showing ?bush-like? features, (c) under
DC driven electrical current showing ?tree-like? features and (d) filling of an
ET grown vascular network with a UV visualization dye.

Electrical treeing (ET) is
the result of partial discharges in a dielectric material. In the vicinity of a
small diameter electrode, the local electric field is greater than the global
dielectric strength, causing a localized, step-wise, breakdown to occur forming
a highly branched interconnected structure (Fig. 2a). The growth of
these structures is influenced by the configuration of the electrodes, with
geometries of a point lead electrode to a point or plane ground electrode being
of most interest. ET is a viable method to produce networks in 2D systems and
in more robust 3D systems on a smaller, micron, scale than the products of the EHVF
method. AC driven electrical current (Fig. 2b), harnessing a sine wave
at 100 Hz, grows a ?bush-like? structure with many branches and therefore a
larger volume within the epoxy samples. DC driven electrical current (Fig. 2c)
produces a more ?tree-like? structure with fewer branches and bifurcations. The
surface of the electrodes were modified with dispersed multi-walled carbon
nanotubes (MWCNTs) to aid in increasing the local electric field, and thus
enable a higher rate of tree initiation and growth. Inclusion of particles was
investigated to determine if the growth direction can be manipulated. The use
of self-clearing electrodes (as a grounding material) was investigated with the
infiltration of a UV dye through the hollow channels produced by ET resulting
in a vascularized network capable of repeated fillings and evacuations. Fluid
delivery (Fig. 2d) can be tailored through the applications of different
electrode and ground manufacturing techniques for optimized flow rates for a
given application.