Spontaneous electrical charging of droplets by conventional pipetting

Abstract

We report that a droplet dispensed from a micropipette almost always has a considerable electrical charge of a magnitude dependent on the constituents of the droplet, on atmospheric humidity and on the coating material of pipette tip. We show that this natural electrification of a droplet originates from the charge separation between a droplet and pipette tip surface by contact with water due to the ionization of surface chemical groups. Charge on a droplet can make it difficult to detach the droplet from the pipette tip, can decrease its surface tension, can affect the chemical characteristics of solutions due to interactions with charged molecules, and can influence the combination and localization of charged bio-molecules; in all cases, the charge may affect results of experiments in which any of these factors is important. Thus, these findings reveal experimental parameters that should be controlled in experiments that use micropipettes.

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“…The first mechanism is governed by the initial charges of the water droplets. Contact electrification of water with a pipe/air [17,30] and with conventional pipettes [31] can cause triboelectric charges in water before the dispersion step. Although the polarity of the triboelectric charge in water can vary by depending on the material that makes contact with the water, it can be inferred from measured data (Fig.…”

“…Although the exact mechanism is still under debate, impurities in the solution can interrupt contact electrification between water and a solid surface. For example, the triboelectric charge amount induced during conventional pipetting is reduced with an increase in the KCl concentration [31]. Even during the contact and slide process of water on the polymer surface, the induced triboelectric charges are reduced by the high impurity concentration.…”

“…Based on this mechanism, most research on TENGs has focused on harvesting ambient mechanical energy types such as vibrations [21][22][23], air flows [24,25] and the mechanical energy of water [26,27]. On the other hand, water-solid interaction also causes triboelectricity with the formation of an electrical double layer (EDL) [28][29][30][31][32]. Using water as a charging material directly, a TENG can harvest water energy (water-TENG) based on a fairly simple structure consisting of one electrode and one solid material [19,20].…”

Slide electrification: charging of surfaces by moving water drops

“…34,35 Furthermore, water drops ejected from a nozzle are usually charged, leaving behind the opposite charge in the nozzle. 6,8,27,[36][37][38] The charge of ejected water drops depends on the surface chemistry of the tube and nozzle, 39 the flow rate, 40 the pH, 41 the salt concentration, 11 and the potential bias on the nozzle. 42 Here we are concerned with drops moving on solid surfaces, i.e., drops with moving three-phase contact lines.…”

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“…Water-solid contact also induces triboelectric charges on surfaces, creating a charged electrical double layer (EDL) [36][37][38][39][40]. Although the exact charging species of the contact-electrification between water and solid surface is still elusive, this charging phenomenon is thought to be governed by selective absorption of ions in water to solid surface [36,37].…”