A hybrid microfluidic chip with electrowetting functionality using ultraviolet (UV)-curable polymer

Electrowetting (EW) is widely used in digital microfluidics for the manipulation of drops sandwiched between two parallel plates. In contrast, demonstrations of closed microfluidic channels enhanced with EW functionality are scarce. Here, we report a simple, low-cost method to construct such microchannels enclosed between two glass plates, each of which comprises electrodes and insulating layers. Our method uses soft imprint lithography with thiolene precursors to design the channel geometry. UV exposure is used to seal the chips permanently and a silanization treatment renders all inner channel surfaces hydrophobic. Compared to earlier polydimethylsiloxane-based designs, this method allows us to make microchannels with smaller dimensions (down to 10 microns), lower aspect ratios (down to height/length = 1/10), and symmetric electrodes both on the top and the bottom of the channel. We demonstrate the new capabilities with two examples: (i) EW-enhanced drop generation in a flow focusing geometry allows precise and continuous control on drop diameter in the range ≈ 1–15 microns while maintaining monodispersity; (ii) EW allows tuning of the excess water pressure needed to displace oil in a microchannel, leading to spontaneous imbibition at EW number η > 0.89

Intracellular particle tracking as a tool for tumor cell characterization \ud

We studied the dynamics of two types of intracellular probe particles, ballistically injected latex spheres and endogenous granules, in tumor cell lines of differerent metastatic potential: breast tumor cells (MCF-7 malignant, MCF-10A benign) and pancreas adenocarcinoma (PaTu8988T malignant, PaTu8988S benign). For both tissue types and for both probes, the mean squared displacement (MSD) function measured in the malignant cells was substantially larger than in the benign cells. Only a few cells were needed to characterize the tissue as malignant or benign based on their MSD, since variations in MSD within the same cell line were relatively small. These findings suggest that intracellular particle tracking (IPT) can serve as a simple and reliable method for characterization of cell states obtained from a small amount of cell sample. Mechanical analysis of the same cell lines with atomic force microscopy (AFM) in force-distance mode revealed that AFM could distinguish between the benign and malignant breast cancer cells but not the pancreatic tumor cell lines. This underlines the potential value of IPT as a complementary nanomechanical tool for studying cell-state-dependent mechanical propertie

Dynamics of ballistically injected latex particles in living human endothelial cells

We studied the dynamics of ballistically injected latex particles (BIP) inside endothelial cells, using video particle tracking to measure the mean squared displacement (MSD) as a function of lag time. The MSD shows a plateau at short times and a linear behavior at longer times, indicating that the BIP are trapped into a viscoelastic network. To reveal more about the molecular constituents and the dynamics of this actin network, we added a variety of drugs. Latrunculin and Jasplakinolide aimed at intervening with the actin network caused a strong increase in MSD, whereas Taxol aimed at microtubules gave no measurable change in MSD. Additional corroborating information about these drug effects were obtained from MSD amplitude and exponent distributions and from fluorescent staining images of the actin and microtubule networks. Our evidence strongly suggests that BIP are primarily embedded in the actin network. Additional drug interventions aimed at disabling non-thermal forces could not conclusively resolve the nature of the forces driving BIP dynamics

Wetting of Mineral Surfaces by Fatty-Acid-Laden Oil and Brine: Carbonate Effect at Elevated Temperature

Oil recovery yields from sandstone reservoirs strongly depend on the wetting properties of the rock. Carboxylic acids present in crude oil may decrease the water wettability by adsorbing onto the mineral surface via cation interactions. A highly simplified version of this scenario has been mimicked in the lab to study these mechanisms in more detail. In previous studies on oil/brine/mineral systems the formation of fatty acid monolayers on mica was observed, yielding water contact angles in ambient oil of up to 60°. Here we demonstrate that the presence of 2 mM bicarbonate (typical for brines) has a strong influence at temperatures above 40 °C (as in reservoirs), yielding water contact angles in ambient oil up to 160°. Similar behavior was found for a variety of carboxylic acids. On increasing the (even) carbon number of simple fatty acids from 8 to 20, the contact angle becomes larger until it saturates at 16 carbon atoms. Similar hydrophobic layers are formed by pulling a sheet of mica through an oil/water interface at comparable velocities. By studying the nanometer-scale topography and chemistry of these dip-coated samples, we can infer that the adsorbed layer is composed of alternating carboxylic acid bilayers that are held together by a very thin intermediate layer containing calcium and (bi)carbonate ions. Exposure to low-salinity water makes the multilayers disappear and the mineral surface become water-wet again, demonstrating that the presence of these structures can lead to a strong salinity-dependent wettability alteration

Detection of ion adsorption at solid–liquid interfaces using internal reflection ellipsometry

We use imaging internal reflection ellipsometry (IRE) in combination with a microfluidic device to study the adsorption of inorganic salt ions to silica–water interfaces. In our data analysis, the measured polarization-dependent reflectivity is compared to calculations from a layer stack model, where the electric double layer is modeled as a separate layer. Due to the high resolution of our technique, we are able to quantify the adsorption of Na+ and Ca2+ ions from aqueous solutions of their chloride salts as a function of their bulk concentrations at pH 3 and 10. Our measurements demonstrate a preferential adsorption of Ca2+ counterions. The experimental results are well described by calculations using a triple layer surface complexation model for the electric double layer with published equilibrium constant

Combined microfluidics–confocal Raman microscopy platform for studying enhanced oil recovery mechanisms

To study the mechanism of enhanced oil recovery, it is important to characterize the three-dimensional spatial distribution of various chemical species, especially water and oil, and their evolution during the course of water flooding. For example, visualizing the (selective) removal of oil from clay or silica substrates by low salinity water can yield important insights. Here, we present a platform that uses a microfluidic device (to represent water flooding at the pore scale) in combination with confocal Raman microscopy. Distributions of oil, water, and minerals are resolved at submicrometer resolution upon flooding water with changing composition. Using glass and gibbsite to mimic sandstone and clay, and water containing divalent cations (Ca 2+ ), we find that oil containing a fatty acid preferentially adsorbs on the gibbsite. Removal of the divalent cations leads to release of the oil droplet. This finding is consistent with the multiple ion exchange mechanism and underlines that the presence of clay is important for low salinity enhanced oil recovery. We expect that our platform will pave the road towards systematic screening of water flood compositions in more complex systems

Bubble formation in catalyst pores: curse or blessing?

H2O2 decomposition experiments on Pt were performed in a glass microreactor, simulating arrays of catalyst pores. The formation of bubbles inside the model nanopores was observed with an optical microscope. It was found that the bubble initiation time strongly depends on the diffusion length and the H2O2 concentration. The amount of catalyst did not have a significant effect, suggesting that the reaction is diffusion limited. Results show that bubble formation can decrease the reaction rate by physically blocking the active sites, but also can accelerate the reaction by creating a forced convective flow inside the nanochannels due to bubble migration. Similar behaviour is likely to occur in a real catalyst and thus, a smart design of the catalytic support could be used to enhance reaction rates

Ion effects in the adsorption of carboxylate on oxide surfaces, studied with quartz crystal microbalance

We chose water-soluble sodium hexanoate as a model organic molecule to study the role of salt ions (Ca2+, Na+, Cl−) in the adsorption of carboxylates to mineral surfaces (silica, alumina, gibbsite) of variable surface charge and chemistry. Quartz crystal microbalance (QCM-D) measurements reveal a qualitatively different dependence of the adsorption behavior on the electrolyte composition for the different surfaces at near neutral pH. Overall, hexanoate adsorption is more pronounced on the positively charged alumina surfaces than on negatively charged silica surfaces. On silica, however, Ca2+ ions strongly enhance the adsorption of hexanoate, suggesting that the divalent cations act as bridges between carboxylate and deprotonated silanol surface groups. On alumina, hexanoate adsorption is found to depend only weakly on the salt composition, suggesting a direct interaction of the carboxylate group with the surface, consistent with a ligand-exchange mechanism. The adsorption behavior on partially gibbsite-covered silica surfaces is particularly rich and displays a strong non-monotonic dependence on the CaCl2 concentration. Comparison to earlier work and control experiments suggest an important role of Cl− anions, which compete with the carboxylate group for adsorption sites