Electrophoretic separations in poly(dimethylsiloxane) microchips using a mixture of ionic and zwitterionic surfactants

pH-Tunable electrokinetic movement of droplets

Manipulation and control of droplet motion in an electric field is of interest in biological systems, microfluidics and electrokinetic (EK) separation techniques. In this work, we show that the electrokinetic motion of oil-in-water (O/W) emulsions stabilized by an amphoteric surfactant can be controlled by changing the pH. Amphoteric surfactants carry both positive and negative head groups and change charge under the influence of changing pH, which allows them to impact the surface charge of droplets as a function of pH, and in extension their direction of motion in an electric field. Using a microfluidic system, we evaluate the effect of pH, surfactant concentration and droplet size on the EK velocity of droplets, which is a combination of electrophoresis (EP) and electro-osmotic flow (EOF). We show that by changing the pH from acidic to alkali, the direction of droplet motion in an external electric field changes. The magnitude of the EK velocity at acidic and neutral pH is not significantly altered as a result of the competition of the EP and EOF in the system, which generally have opposite directions. Our results are in good agreement with theoretical predictions for the droplet EP mobility and can thus serve as a verification of the theoretical descriptions. In addition to the pH, the surfactant concentration affects droplet EK velocity, most specifically at pH of 7 which is close to the isoelectric point of the surfactant monomers. At this pH, changing the surfactant concentration changes the direction of droplet motion due the competing effect of the EP and EOF at different surfactant concentrations. By increasing the droplet size, the magnitude of the EK velocity increases because of the larger local ζ-potential of the larger droplets as well as the wall-enhanced effect in the system. The results from this work can be applied to design on-chip droplet separation strategies based on pH variations and are relevant for systems in which pH gradients naturally occur, such as the human body.

Zwitterionic surfactant as an additive for efficient electrophoretic separation of easily absorbed rhodamine dyes on plastic microchips

Plastic microchips possess the advantages of easy fabrication and low-cost, but their surface properties are frequently incompatible with electrophoretic separation without proper surface modification. Meanwhile, the separation microchannels on typical microchips are usually only a few centimeters long, the pressurized flow may significantly affect the electrophoretic separation if their inner diameters (id) are relatively larger (approximately > 50 μm), viscous separation medium is therefore required for efficient separation. Herein, a zwitterionic surfactant, N-hexadecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (HDAPS), was used as a multifunctional additive to inhibit the analyte adsorption, improve the surface status, control Joule heating and modulate the resolution on cyclic olefin copolymer microchips with 80 μm id, 5 cm long separation microchannels, eliminating the necessity of viscous polymeric additives. The effectiveness of HDAPS was compared with an ionic polymeric additive, poly(diallydimethylammonium chloride). The streaming potential and electroosmotic flow measurements indicated an effective inhibition of the adsorption of rhodamine B and a stable negative surface charge with zwitterionic HDAPS. Using 15 mmol/L HDAPS, 40% (v/v) methanol, and 10 mmol/L boric acid (pH 3.2) as the running buffer, rapid separation of four rhodamines was achieved within 90 s under a separation electric field of 520 V/cm. The theoretical plate numbers were in a range of 5.0×10⁵-6.9×10⁵/m. The relative standard deviations were no more than 0.9% for retention time and 1.5% for peak area. The proposed system was verified by the determination of rhodamines in eyeshadow and wolfberry, with standard recoveries in a range of 98.2%-101.4%.

Stability of Two-Dimensional Liquid Foams under Externally Applied Electric Fields

Liquid foams are highly complex systems consisting of gas bubbles trapped within a solution of surfactant. Electroosmotic effects may be employed to induce fluid flows within the foam structure and impact its stability. The impact of external electric fields on the stability of a horizontally oriented monolayer of foam (2D foam) composed of anionic, cationic, non-ionic, and zwitterionic surfactants was investigated, probing the effects of changing the gas-liquid and solid-liquid interfaces. Time-lapse recordings were analyzed to investigate the evolution of foam over time subject to varying electric field strengths. Numerical simulations of electroosmotic flow of the same system were performed using the finite element method. Foam stability was affected by the presence of an external electric field in all cases and depended on the surfactant type, strength of the electric field, and the solid material used to construct the foam cell. For the myristyltrimethylammonium bromide (MTAB) foam in a glass cell, the time to collapse 50% of the foam was increased from ∼25 min under no electric field to ∼85 min under an electric field strength of 2000 V/m. In comparison, all other surfactants trialed exhibited faster foam collapse under external electric fields. Numerical simulations provided insight as to how different zeta potentials at the gas-liquid and solid-liquid interfaces affect fluid flow in different elements of the foam structure under external electric fields, leading to a more stable or unstable foam.

Fused filament fabrication 3D printed polylactic acid electroosmotic pumps

Additive manufacturing (3D printing) offers a flexible approach for the production of bespoke microfluidic structures such as the electroosmotic pump. Here a readily accessible fused filament fabrication (FFF) 3D printing technique has been employed for the first time to produce microcapillary structures using low cost thermoplastics in a scalable electroosmotic pump application. Capillary structures were formed using a negative space 3D printing approach to deposit longitudinal filament arrangements with polylactic acid (PLA) in either "face-centre cubic" or "body-centre cubic" arrangements, where the voids deliberately formed within the deposited structure act as functional micro-capillaries. These 3D printed capillary structures were shown to be capable of functioning as a simple electroosmotic pump (EOP), where the maximum flow rate of a single capillary EOP was up to 1.0 μl min^-1 at electric fields of up to 750 V cm^-1. Importantly, higher flow rates were readily achieved by printing parallel multiplexed capillary arrays.

Mechanistic studies of droplet electrophoresis: A review

Electrophoresis (EP) of droplets is an intriguing phenomenon that has applications in biological systems, separation strategies, and reactor engineering. Droplet EP is significantly different from the classic particle EP because of droplet characteristics such as a mobile surface charge and the nonrigidity of the interface. Also, the liquid-liquid system, where there is an interplay between the hydrodynamic and electrokinetic forces in both phases, adds to the complexity of electrophoretic motion. Due to the vast amount of potential applications of droplet EP, a mechanistic understanding of the droplet motion in the presence of an external electric field is crucial. This review provides a background on the mechanism of droplet EP and summarizes the intrinsic interplay between the different relevant forces in these systems. The review also describes the key differences between droplet EP and particle EP, and the impact of these differences on droplet mobility. Additionally, we schematically summarize the effects of key parameters on droplet EP mobility, such as electric double layer polarization, the development of internal flow inside a droplet and boundary effects.

Observation of Dynamic Surfactant Adsorption Facilitated by Divalent Cation Bridging

Dynamic evidence of the mechanism for surfactant adsorption to surfaces of like charge has been observed. Additionally, removal and retention of surfactant molecules on the surface were observed as a function of time. A decrease in surface charge is observed when metal counterions are introduced and is dependent on charge density as well as valency of the metal ion. When surfactant species are also present with the metals, a dramatic increase in surface charge arises. We observed that the rate and quantity of surfactant adsorption can be controlled by the presence of divalent Ca²⁺. Under isotonic conditions the introduction of Ca²⁺ is also easily distinguishable from that of monovalent Na⁺ and provides dynamic evidence of the divalent "cation bridging" phenomenon. Dynamic changes to surface charge are experimentally determined by utilizing current monitoring to quantify the zeta potential in a microfluidic device.

Zeta Potential of Poly(methyl methacrylate) (PMMA) in Contact with Aqueous Electrolyte-Surfactant Solutions

The addition of surfactants can considerably impact the electrical characteristics of an interface, and the zeta potential measurement is the standard method for its characterization. In this article, a comprehensive study of the zeta potential of poly(methyl methacrylate) (PMMA) in contact with aqueous solutions containing an anionic, a cationic, or a zwitterionic surfactant at different pH and ionic strength values is conducted. Electrophoretic mobilities are inferred from electrophoretic light scattering measurements of the particulate PMMA. These values can be converted into zeta potentials using permittivity and viscosity measurements of the continuous phase. Different behaviors are observed for each surfactant type, which can be explained with the various adsorption mechanisms on PMMA. For the anionic surfactant, the absolute zeta potential value below the critical micelle concentration (CMC) increases with the concentration, while it becomes rather constant around the CMC. At concentrations above the CMC, the absolute zeta potential increases again. We propose that hydrophobic-based adsorption and, at higher concentrations, the competing micellization process drive this behavior. The addition of cationic surfactant results in an isoelectric point below the CMC where the negative surface charge is neutralized by a layer of adsorbed cationic surfactant. At concentrations near the CMC, the positive zeta potential is rather constant. In this case, we propose that electrostatic interactions combined with hydrophobic adsorption are responsible for the observed behavior. The zeta potential in the presence of zwitterionic surfactant is influenced by the adsorption, because of hydrophobic interactions between the surfactant tail and the PMMA surface. However, there is less influence, compared to the ionic surfactants. For all three surfactant types, the zeta potential changes to more-negative or less-positive values for alkaline pH values, because of hydroxide adsorption. An increase of the ionic strength decreases the absolute value of the zeta potential, because of the shielding effects.

Electrophoretic separations in poly(dimethylsiloxane) microchips using mixtures of ionic, nonionic and zwitterionic surfactants

The use of surfactant mixtures to affect both EOF and separation selectivity in electrophoresis with PDMS substrates is reported, and capacitively coupled contactless conductivity detection is introduced for EOF measurement on PDMS microchips. First, the EOF was measured for two nonionic surfactants (Tween 20 and Triton X-100), mixed ionic/nonionic surfactant systems (SDS/Tween 20 and SDS/Triton X-100), and finally for the first time, mixed zwitterionic/nonionic surfactant systems (TDAPS/Tween 20 and TDAPS/Triton X-100). EOF for the nonionic surfactants decreased with increasing surfactant concentration. The addition of SDS or TDAPS to a nonionic surfactant increased EOF. After establishing the EOF behavior, the separation of model catecholamines was explored to show the impact on separations. Similar analyte resolution with greater peak heights was achieved with mixed surfactant systems containing Tween 20 and TDAPS relative to the single surfactant system. Finally, the detection of catecholamine release from PC12 cells by stimulation with 80 mM K(+) was performed to demonstrate the usefulness of mixed surfactant systems to provide resolution of biological compounds in complex samples.