# Charge induced deformation of scanning electrolyte before contact

Liang Liu

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## To cite this version:

Liang Liu. Charge induced deformation of scanning electrolyte before contact. Faraday Discussions, Oct 2024, Nanjing (China), China. http://dx.doi.org/10.1039/D4FD0000000

HAL Id: hal-05098049

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Submitted on 4 Jun 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Charge induced deformation of scanning electrolyte before contact

Liang Liu

Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l'Environnement (LCPME),
Université de Lorraine, CNRS, Nancy 54000, France

liang.liu@cnrs.fr

Scanning electrochemical probe techniques have been constantly developing in the last decades. The recent trend is based on scanning the electrolyte, either in liquid or gel form. In experimental practice, the electrolyte is filled in a pulled capillary, or deposited on metal electrodes (for gel) obtaining the probe. For measuring spatially localized electrochemistry, the probe is approached to be in contact with the sample, and may also laterally scan over the sample by repeating the intermittent contact. The strategy of scanning electrolyte (scanning electrochemical cell microscopy (SECCM), etc.) pushes the spatial resolution of scanning electrochemical probe techniques into nanoscale without sophisticated probe fabrication, which is a main advantage over classical scanning electrochemical microscopy.

The most straightforward way to sense the contact between the electrolyte probe and the sample surface is by current feedback. By applying potential between the probe and the sample, a current spike will turn up upon their contact due to the bridging of the circuit. This signal is highly useful for the instrumentation, while the electrochemistry is usually analyzed by the measurements afterwards (could be in the order of milliseconds). Yet, a question arises out of curiosity: What is the physical meaning of this current at very first contact?

In this work, I attempt to look into this problem from the theoretical aspect. Considering that in scanning electrochemical probe techniques, the electrolyte is usually well supported for reasonably high conductivity, so it can be approximately treated as a perfect conductor. However, the electrolyte will be very close to the sample right before contact, thus its geometry will clearly affect the distribution of electric field. These are very different from electrohydrodynamic methods (electrospinning, electrospraying, even at near-field). A simple estimation can be derived from the problem illustrated in **Fig. 1**, where a hemispherical, rigid, perfectly conductive body mimicking electrolyte is positioned close to an infinitely large perfectly conductive planar sample connecting to ground. When a constant potential is applied on the hemisphere, the potential distribution can be solved with analytical solution. Based on this, one may calculate the charge cumulated in the hemisphere and then the electric force by Gauss Law. It can be seen that the pressure induced by the electric force may be much higher than the gravity, surface tension, or Young's modulus (in the case of gel) under reasonable experimental conditions. That being said, if the hemisphere is not rigid (which is the real case), it will deform.

To solve this problem, the ongoing work starts with an approximation considering the hemisphere as an elastic body. This may be valid for the gel electrolyte when approaching at a low speed. The strategy is to take the analytical solution from rigid body as initial value, then calculate the deformation based on the electric field, recalculate the electric field based on deformed geometry until reaching convergence. When the convergence cannot be achieved, it is likely suggesting the contact between the electrolyte probe and the sample. The current can then be calculated from the discharging, and factors affecting it will be explored. Depending on the progress, more sophisticated cases may also be demonstrated in the full paper and/or in the conference. I believe that the better understanding of physical and chemical processes at the moment of electrode-electrolyte contact is a fundamental interest, and may also improve the spatial resolution of the measurements if well controlled experimentally.

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Assume a = 1 µm, d = 100 nm, $\varphi_{appl}$ = 1 V

Maximum electrical stress: 1.2 kPa

<caption>**Fig. 1.** Approximation of a hemispherical, rigid, perfectly conductive body mimicking electrolyte approaching an infinitely large perfectly conductive planar sample connecting to ground.</caption>

## Acknowledgement

The author acknowledges the financial support from ANR-NRF project MEACT.