Ferroelectric-nematic droplets moved in only a minority of the reported trials. Under fresh PTFE conditions, motion occurred in approximately 30% of attempts, while the remaining 70% showed deformation or electromechanical instability. Motion was reported only while the droplets were in the ferroelectric-nematic phase. When they responded to the sign of the surrounding charge, they were repelled by positive charges and attracted to negative ones, consistent with behaving as if they were positively charged.
The observations came from a laboratory setup in which RM734 droplets were deposited on PTFE-coated glass coverslips near lithium-niobate crystals. During heating and cooling cycles, the setup exposed them to strongly non-uniform, or fringing, electric fields - fields that changed sharply across a short distance - while uniform or nearly uniform fields were used for comparison. In the reported cooling protocol, the temperature was lowered to 105°C at an average rate of 0.1°C per second in the ferroelectric range. A polarizing optical microscope and a CCD camera recording at 25 frames per second monitored the droplets.
The field geometry marked a clear contrast
In the comparison tests, no droplet motion was observed under the tested uniform electric field. With two lithium-niobate crystals whose polar axes pointed in opposite directions, the field was quasi-uniform, and no motion was observed regardless of droplet position. The result was consistent with the authors' proposed requirement for highly non-uniform fields for the charging response.
Under the quasi-uniform field, the droplets were observed to undergo significant elastic deformation. The extent of the shape change depended on the square of the field, and some droplets split, or underwent fission. The authors interpreted this as electrostriction-like deformation linked to electrical polarization.
One reported sequence offered a clue about timing. A droplet first moved in the direction the paper attributed to dielectrophoresis, a field-gradient-driven motion, and then began moving away after tens of seconds. The authors interpreted that delayed reversal as charging after exposure to the non-uniform field. The sequence was described as rare, and its frequency was not quantified.
Speed also varied between measurements. Average droplet speed was higher during heating and when the lithium-niobate polar axis was negative, although the text did not give exact speeds or their spread. The researchers estimated velocity from the time derivative of the droplet-position curve and used an approximate balance with viscous friction to interpret the force.
The charge explanation remains provisional
The authors' proposed explanation centers on splay, a local distortion of polarization inside a droplet. They argue that a highly non-uniform field can create localized polarization-charge buildup at the PTFE interface, and that contact electrification - charge exchange when the droplet and coating touch - minimizes the accumulated charge. In this account, the apparent positive-charge behavior is an interpretation of the motion, not a direct measurement of net charge or electron-transfer events.
The proposed direction of electron transfer was also examined through a calculated RM734 work-function value and a reported PTFE value. RM734's value was 8.47 electronvolts, lower than PTFE's 11.0 eV. The molecular calculation used Gaussian 16 Rev. B.01, with geometry optimization at B3LYP-D3(BJ)/6-311++G(d,p) followed by harmonic-frequency analysis. That comparison is suggestive only: it does not directly measure charge transfer at the RM734-PTFE interface.
What the pattern does not settle
The positive-charge interpretation therefore rests on motion and its response to field sign, rather than on a direct charge measurement. The study did not quantify net droplet charge, transferred charge or electron-transfer events. It also reports that motion varied with droplet size, initial distance, PTFE coating condition, pinning and other experimental factors, several of which were not quantified. Total attempts and condition-specific sample sizes were not reported, so the approximately 30% motion figure is descriptive and carries no stated statistical uncertainty.
Taken together, the findings describe a conditional association: highly non-uniform fields coincided with occasional motion showing a positive-charge signature, while the tested uniform field showed no motion and the antiparallel two-crystal configuration was associated with deformation and, sometimes, fission. The evidence is limited to RM734 droplets on PTFE-coated glass in this lithium-niobate laboratory geometry. It does not establish electron transfer as the source of the motion or reproducible quantitative control of droplet movement.
The authors say the research was sponsored by the Army Research Laboratory under Grant Number W911NF-24-1-0252, and they declare no conflict of interest.
Paper data and sources
Original title: Splay-induced charging of ferroelectric droplets in highly non-uniform electric fields
Authors: Lorenzo Fiorentini, Elia Rocchetti, Raouf Barboza et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text