Small amounts of water can enter diesel fuel during usage, causing major damage and failure of engine parts. Water is dispersed in fuel as droplets stabilized by the presence of surface-active compounds in the original fuel mixture as well as in fuel additives, including lubricity improvers and deposit control agents. Additives partition to the fuel–water interface and lower the interfacial tension (IFT), decreasing the ability to coalesce and separate water from fuel. The ability of standard coalescing filters to capture and coalesce emulsion droplets depends on dynamic IFT, conventionally measured for large millimeter-sized drops or planar interfaces. In this work, a microfluidic platform is employed to generate a monodisperse stream of small micrometer-sized water droplets in model fuel and ultralow sulfur diesel, mimicking the size of droplets in actual fuel–water emulsions. The deformation of hundreds of droplets is tracked at high speed through twenty-six geometric contractions to find time-dependent apparent IFT. It is found that the time scale associated with the decrease of IFT is orders of magnitude smaller in micrometer-sized droplets compared to millimeter-sized drops from pendant drop experiments. This finding suggests that, in real emulsion processing conditions such as fuel filtration, the residence time of droplets from the point of formation to filtration is such that IFT has already decreased to the equilibrium value. This work results in clear implications that standardized tests used by industry for qualifying diesel fuels must be reconsidered to account for droplet size, to enable design of efficient fuel filtration systems.
Coalescence of micrometer-scale droplets is impacted by several parameters, including droplet size, viscosities of the two phases, droplet velocity, angle of approach, as well as interfacial tension and surfactant coverage. The thinning dynamics of films between coalescing droplets can be particularly complex in the presence of surfactants, due to the generation of Marangoni stresses and reduced film mobility. Here, a microfluidic hydrodynamic "Stokes" trap is used to gently steer and trap surfactantladen micrometer-sized droplets at the center of a cross-slot. Water droplets are formed upstream of the cross-slot using a microfluidic T-junction, in heavy and light mineral oils and stabilized using SPAN 80, an oil-soluble surfactant. Incoming droplets are made to coalesce with the trapped droplet, yielding measurements of the film drainage time. Film drainage times are measured as a function of continuous phase viscosity, incoming droplet speed, trapped droplet size, and surfactant concentrations above and below the critical micelle concentration (CMC). As expected, systems with higher surfactant concentrations and slower incoming droplet speed exhibit longer film drainage times. At low surfactant concentrations, the drainage time is longer for the more viscous heavy mineral oil in the continuous phase, whereas at high surfactant concentrations, the dependence on continuous phase viscosity vanishes. Perhaps more surprisingly, larger droplets and high confinement also result in longer film drainage times, potentially due to deformation of the droplet interfaces. The results are used here to determine critical conditions for coalescence, including both an upper and a lower critical capillary number. Moreover, it is shown that induced surfactant concentration gradient effects enable coalescence events after the droplets had originally flocculated, at surfactant concentrations above the CMC. The microfluidic hydrodynamic trap provides new insights into the role of surfactants in film drainage and opens avenues for controlled coalescence studies at micrometer length scales and millisecond time scales.
The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.
The luminescent tungsten-alkylidyne metalloligand [WCl(≡C-4,4'-C6H4CC-py)(dppe)2] (1; dppe=1,2-bis(diphenylphosphino)ethane) and the zinc-tetraarylporphyrins ZnTPP and ZnTP(Cl)P (TPP=tetraphenylporphyrin, TP(Cl)P=tetra(p-chlorophenyl)porphyrin) self-assemble in fluorobenzene solution to form the dyads ZnTPP(1) and ZnTP(Cl)P(1), in which the metalloligand is axially coordinated to the porphyrin. Excitation of the porphyrin-centered S1 excited states of these dyads initiates intramolecular energy-transfer (ZnPor→1) and electron-transfer (1→ZnPor) processes, which together efficiently quench the S1 state (~90%). Transient-absorption spectroscopy and an associated kinetic analysis reveal that the net product of the energy-transfer process is the (3)[dπ*] state of coordinated 1, which is formed by S1→(1)[dπ*] singlet-singlet (Förster) energy transfer followed by (1)[dπ*]→(3)[dπ*] intersystem crossing. The data also demonstrate that coordinated 1 reductively quenches the porphyrin S1 state to produce the [ZnPor(-)][1(+)] charge-separated state. This is a rare example of the reductive quenching of zinc porphyrin chromophores. The presence in the [ZnPor(-)][1(+)] charge-separated states of powerfully reducing zinc-porphyrin radical anions, which are capable of sensitizing a wide range of reductive electrocatalysts, and the 1(+) ion, which can initiate the oxidation of H2, produces an integrated photochemical system with the thermodynamic capability of driving photoredox processes that result in the transfer of renewable reducing equivalents instead of the consumption of conventional sacrificial donors.
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