We study the influence of electrowetting on the formation of undesired solute residues, so-called coffee stains, during the evaporation of a drop containing non-volatile solvents. Electrowetting is found to suppress coffee stains of both colloidal particles of various sizes and DNA solutions at alternating (AC) frequencies ranging from a few Hertz to a few tens of kHz. Two main effects are shown to contribute to the suppression: (i) the time-dependent electrostatic force prevents pinning of the three phase contact line and (ii) internal flow fields generated by AC electrowetting counteract the evaporation driven flux and thereby prevent the accumulation of solutes along the contact line.

Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow. The experiments are carried out with a counterrotating Couette apparatus. The deformation depends on the mechanical properties of the lipid bilayer, the vesicle radius, and the viscosity of the surrounding Newtonian liquid. We show that the relevant mechanical parameter is the bending rigidity. A simple model has been developed that describes the deformation of a vesicle. This model takes thermal undulations of the bilayer into account. We have obtained a value for the bending rigidity of dimyristoyl-phosphatidylcholine bilayers and its value has been compared with literature data and with results from micropipette aspiration experiments. From the measurements we are able to discriminate between unilamellar and multilamellar vesicles.

The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca²⁺ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (≈10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K⁺ and Na⁺ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca²⁺. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology.

We integrate a sensitive microfluidic comparator into a T-junction device and report measurements of the excess pressure drop due to a single moving droplet confined in a rectangular microchannel. We specifically focus on drops that are not coated with surfactants and study the effects of drop size, droplet viscosity and capillary number on their hydrodynamic resistance. In the capillary number range of ≈ 10⁻³–10⁻², we find two distinct regimes for hydrodynamic resistance behavior based on drop size. In regime I associated with small drops (drop length/channel width ∼ ∼4), depending on the viscosity ratio of the two phases, the hydrodynamic resistance could increase, decrease or remain unchanged with drop size. We present a simple model that qualitatively captures these experimental trends. This model reveals that the pressure drop in regime I is dominated by the dissipation due to the end caps, and in regime II by both the end caps and the central body of the droplet. Such fundamental understanding will enable the design of large-scale energy-efficient fluidic circuits by minimizing the overall pressure drop in a network and may also provide insights into controlling droplet traffic to build functional passively-driven two-phase microfluidic technologies.