Instrumentation & Measurement Magazine 25-9 - 7

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1
-1
V
I
Separation
0 0.1 0.2 0.3 0.4 0.5
Time (s)
(a)
(b)
Fig. 4. (a) Typical current (red) and voltage (blue) signal measured by an electrometer in the plant tissue when a material (here silicone rubber) repeatedly
touches the leaf. The grey line displays the contact and separation distance between the leaf and the material touching it. Only when the two materials carrying
the charges generated by triboelectrification are mechanically separated, the electrostatic induction leads to significant electrical charges in the tissue. (b) The
mechanism of leaf contact-/triboelectrification and electrostatic induction corresponding to the peaks highlighted by the numbered circles in a). Figures adapted
from [1], ©2018 Wiley, with permission.
contacting the leaf surface and separating again from it before
starting the next cycle. This contact-release motion creates
an alternating current/voltage (Fig. 4a). Fig. 4b displays the
mechanism leading to the potential differences in the tissue
consisting of the triboelectric charging upon contact and the
electrostatic induction of the charges in the leaf cellular tissue.
The signal's polarity and the amplitude give information
on the charges created on the leaf surface and that formed on
the contacting material. As leaves generally charge positively
during contact electrification, especially the amplitude gives
information on the capability of which materials are suited
to charge the cuticle by triboelectrification. The amplitude of
voltage signals from single leaves can reach up to over 150 V
(leaf size of only 4.5 cm2
the μA range [1]. The peak power output is 15 μW per cm2
) and current signals are typically in
leaf
contact area measured at a load resistance of ~250 MΩ from a
single impact of silicone rubber with a Rhododendron leaf (impact
force 1 N). The typical peak duration is ~15 ms (full width
at half maximum), resulting in a harvested electrical energy
of 225 nJ each time the leaf and the silicone rubber touch each
other [1]. As the leaf-artificial material combination resembles
a triboelectric nanogenerator (TENG), models that have been
reported for TENGs can be used to simulate and to some extent
to predict the voltage output. In such models, the output
voltage signal is described by considering two charged and
moving capacitor plates. Maxwell-equations can be used to
describe the electric displacement field. When a load is connected
to the circuits, the output voltage due to a transient
mechanical load is described by the relation V = R*dQ/dt
where Q, the transferred charge, can be obtained by an exponential
function that assumes a certain surface charge density
on the two contacting surfaces. Further in-depth description of
the model can, for example, be found in [9].
Advanced Analysis of Plant Leaf Energy Conversion
The details of the mechanism of surface charging including, for
example, charge distributions on leaves are more challenging
December 2022
to investigate. One opportunity that is often used in the analysis
of static charges on artificial materials is Kelvin Probe Force
Microscopy (KPFM) [10]. The measurement technique is based
on atomic force microscopy (AFM) and can give simultaneous
information on the surface micro/nanoscale topography
and the surface charges. Leaves are not the ideal sample to
perform such analysis due to their intrinsic roughness which
makes scanning of the surface with a nanosized cantilever difficult.
Yet, some species that have flatter surfaces are suitable
for such analysis. The surface charge on Rhododendron leaves
could be measured before and after a sample of the leaf has
been touched by another material of interest [1]. It is important
to scan exactly the same area before and after treatment, and
some KPFM/AFM instruments are able to perform a sample
treatment in a way that exactly the same area is scanned after
sample modification with and without the use of markers.
It is especially interesting to analyze leaf surfaces in different
conditions of the underlying tissue. For example, drying the
tissue and thus removing its ability to act as an electrode enables
surface charge patterns on the leaves to be measured and
the kinetics of which static charges vanish (more details are
available in [1]). Such analysis can give more insights into the
mechanisms of leaf surface charging by triboelectricity. However,
tackling yet unanswered questions on the mechanism of
triboelectric charge generation and analyzing related charge
transfer, etc. is clearly easier (though far from trivial) on wellknown,
homogenous artificial materials instead of on highly
complex living leaf surfaces.
Further advanced analysis of plants' electrical signals can
give information on their capability for energy harvesting,
for example, analyzing the mechanical energy conversion
efficiency and the power output as a function of the load resistance
of a leaf-artificial material pair. The energy conversion
efficiency considers the input kinetic energy of an impact between
the two surfaces and the resulting electrical energy.
Thereby, it is important to exactly know these two parameters
and to be aware of other factors which may influence
IEEE Instrumentation & Measurement Magazine
7
Fixed static net
Charges
4
3
2
1
Leaf contact electrification
1
Material in contact
with leaf
Contact
Electrostatic induction and varying capacitance
2
Release
Approach
3
Plant tissue
eMobile,
induced
Charges, ions
eTissue
electrode
Normalized V, I signals
Separation distance (mm)

Instrumentation & Measurement Magazine 25-9

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