Charge of water droplets in non-polar oils

Abstract

Recent advances in droplet manipulation methods by electric fields and signals require a deeper understanding of water droplet charge. In this paper, we have investigated the electrophoretic motion of individual water microdroplets injected into non-polar silicone and paraffin oil by video optical microscopy on an individual droplet basis to determine droplet charge. It was found that the initial surface charge density of surfactant free droplets directly after injection from a micropipette is positive and of the order of 10−6 C/m2, regardless of pH and ion concentration in the range from pH 4 to pH 10 and from 0.01 mmol/l to 1.5 mol/l, respectively. The experimental results together with molecular dynamics simulations show that the nature and polarity of the charge can be explained by anisotropic orientation of water molecules at the interface rather than selective adsorption of ions. Furthermore, we showed that slip at the liquid-liquid boundary must be taken into account when interpreting electrophoretic measurements of droplets.

Electrophoretic manipulation of multiple-emulsion droplets

“…Furthermore, small, charged oil cores can be seen as electrostatically equivalent to globular proteins or cells encapsulated in the water droplets. This means that it is unlikely that the addition of charged particles or cells into water microdroplets would greatly affect any electrostatic manipulation procedures in microfluidic devices, which is compatible with previous results, where we did not see any significant influence of electrolyte ions on the electrophoretic mobility of W/O droplets (Schoeler et al, 2013).…”

“…On the other hand the droplet velocity appears to be of similar magnitude after the shell has made contact with a biased negative electrode regardless of droplet core/shell radius ratio (Fig. 3) (Schoeler et al, 2013). With the addition of the anionic surfactant SDS (2.30 g/l) it was possible to create more stable core-shell droplets and as a result a droplet radius ratio of up to 0.93 was achieved (Fig.…”

“…Once injected, the coreshell droplets sink to the bottom of the cuvette with a gravitational terminal velocity, v g . When a droplet reaches the middle between the Cu electrodes, the voltage, U, is applied, creating an electric field, E, that causes the droplet carrying an effective charge, Q, to be displaced with a horizontal velocity component, v el (Schoeler et al, 2013). The velocity of the droplets was measured in the middle of the cuvette to minimize any sidewall effects and where the terminal velocity, v el , has been reached (Fig.1a).…”

“…All micro-electrophoretic experiments were performed using the same transparent, electrophoretic cell and experimental method as described in (Schoeler et al, 2013) (Fig.1a). Individual oil-in-water droplets (deionized water, density ρ water = 1000 kgm -3 , Millipore Direct-Q 3, Ultrapure Water Systems, Watford, UK) were injected into the continuous silicone oil phase (100 cSt silicone oil (C 2 H 6 OSi) n , dynamic viscosity η silicone = 0.106 Pas, density ρ silicone = 965 kgm -3 electrical resistivity δ silicone = 10 15 Ωcm, dielectric permittivity ε silicone = 2.43×10…”

Contact Charging and Electrphoresis of a Glassy Carbon Microsphere

“…에서 활발히 연구되고 있다 [1][2][3][4][5][6][7][8][9][10][11][12]. 액적 접촉충전 현상 기반의 미세 유체 기술은 개별 액적을 전기를 이용해 다루는 디지털 미세유체 기 술의 한 분야로 [13,14], 채널 내 액적을 다루는 액적 미세유체 (Droplet Microfluidics) [15,16] 기술과는 다르며 기존 미세유체 기술 들의 단점을 극복할 수 있어 새로운 대안 기술로 제시되고 있는 신 기술 이다 [17,18].…”