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1. ABSTRACT
The physical and chemical properties of the water/air interface are of great importance to fundamental science, medicine, and technology. [1-2]. Intrinsic to the interface is the
asymmetrical environment experienced by the interface charges made up of electrons or ions. This unique environment has properties that surely differ from the bulk phase. To shed
light on the electrical properties of this interface, we first studied the effect of applying an electric potential to a metal ring electrode at which center a falling water droplet formed. The
induced charges on the droplets were measured at a Petri dish some centimeters below. In a second experiment the charges induced on the petri dish surface by a square wave
electrical potential applied to the ring electrode were investigated.
When circuit (c) is measuring charge, S2 is OFF. In this case, when S1 switches OFF, the
positive charged cupper ring induces negative charges in the self capacitance of the
isolated water/air surface and positive charges on the self capacitance of the isolated
water/glass surface. The positive charges are discharged to ground through circuit (c) . As
a consequence a negative potential is registered (curve 2, Fig 4-1) and the dish is
negative charged, see Fig 4 inset 2. When S1 switches ON the dish surface discharges to
ground trough (c). A positive potential is registered , curve 2, Fig 4-1.
We investigated the time to induce the charges separation of the surfaces self
capacitances for three solutions of different concentrations of NaCl. S1 switches ON
rapidly (~100ns). We used the ON switching time to measure the time response of the
induced charge, see Fig. 5. The time constants were inversely proportional to the NaCl
concentrations and the induced charges were independent of the NaCl concentration. The
maximum potential of the ring on the water surface, at the distance of 6.2cm was ~100V.
Calculating the self capacitance of a metal plate with the same radius of the petri dish
(7.5cm) gives C=4.1pF. According to Fig 5 the charge induced was ~300pC. This value
divided by the self capacitance gives 73V , close to the calculated 100V.
Conclusion
The droplet water surface charges like a metallic sphere with the same radius by the
presence of a positively charged potential region. The electric charges were induced
on the water surface. Friction was not responsible for the formation of the charges .
In a petri-dish, charge separation time between the air and the glass surfaces
depended on the ions concentration. The induced charges depended on the self-
capacitance of the surfaces.
Acknowledgements
The authors are grateful to J. R. Castro and for technical assistance, to the funding support
of FAPESP.
Experimental: 1- Water Drops Experiment: Figure 1 illustrates the setup
used. Initially a high voltage V was applied to the cupper ring, (e). Switch S1 was off and
Fig. 1 – Experimental setup
Measurements and discussion: Figure 2 shows the droplet charging process
in four different situations:
In situation (1), the reservoir was grounded and a voltage V=0.5kV was applied to the
cupper ring. . The droplets formed in the center of the ring fallen in to the petri dish (c).
Circuit (d) registered a charge of ~Q= - 70pC per droplet.
In situation (2) , the grounding wire was retired. Circuit (d) registered a gradual decrease
of the droplets electric charge to zero Coulombs, shown in situation (3).
In situation (4), the cupper ring is grounded, V=0V. Circuit (d) registered a initial positive
charge of Q= +70pC per droplet, with a gradual decrease to zero Coulombs .
The photographed droplets, Fig 1-a showed a droplet diameter of ~3.0mm.
We compared the electrical potential of a metallic sphere with a spherical water droplet
with the same radius. The electrical potential of a metal sphere is given by the following
equation: ∅ =
𝑄
4𝜋𝜀0 𝑟
where 0 is the vacuum permittivity and r the sphere radius. For the
water droplet radius and for the measured charge of -70pC, the calculated potential was
∅ = −0.49𝑘𝑉, very close to the inverted potential applied to the ring electrode (+0.5kV).
It shows that the droplet surface charges like a metallic sphere with the same radius.
The induced charge was found to be proportional to the voltage applied to the ring
electrode and of opposite sign, see Fig. 3. The self-capacitance of the isolated droplet is
the relation between charge and potential: 𝐶 = 4𝜋𝜀0 𝑅 ≅ 0.16𝑝𝐹.
David Mendez Soares, Juracyr Ferraz Valente Filho, Alexandre Cyrino Lucena, Thiago Virgilio da Silva, Paula Simões
Casagrande and Omar Teschke
SURFACE CHARGES ON WATER/AIR INTERFACES DERIVED FROM
ELECTROSTATIC POTENTIAL
Fig. 2. Different situations for the charging of the water droplets.
Laboratório de Nanoestruturas e Interfaces, Instituto de Física “Gleb Wataghin”, UNICAMP, 13083-859
Campinas, São Paulo, Brazil.
Fig. 4. Experimental setup. Insets 1 and 2 shows surface charging. At the right, Figures 1 and 2
show: curves 1 is the ON-OFF signal of S1. Curve 2 the corresponding potential. .
switched 10 times per second and S2 switched on and off each 0.5s. Circuit (c) is the
charge sensor.
Fig. 3. Charge measured vs Potential applied to the ring
S2 toggled between on
and off each 0.5s.
Circuit (d) is a charge
sensor, Vout= -1.0nC/V.
Charged water drops
from reservoir (a) fallen
in a petri dish (c). Those
fallen drops were
expected to be charged
by the ring potential as
shown in detail (b).
References
1) Zimdars, D.; Eisenthal, K. B.. J. Phys. Chem. B 2001, 105, 3993-4002
2) Musumecia, F; Pollack, G. H.. Chemical Physics Letters, 2014, 613, 19–23
Fig. 5. Different constant times for each concentration of NaCl
Fig. 1 – a) Water drop
photography
2- Charge induction on the water surface of a Petri dish:
Figure 4 illustrates the setup used. Initially a voltage V=0.5kV was applied to the cupper
ring, (a), set at 6.2cm distance from the petri dish water surface (b). Switch S1