In droplet-based ("digital") microfluidics, liquid droplets in contact with dielectric surfaces are created, moved, merged and mixed by applying AC or DC potentials across electrodes patterned beneath the dielectric. We show for the first time that it is possible to manipulate droplets of organic solvents, ionic liquids, and aqueous surfactant solutions in air by these mechanisms using only modest voltages (<100 V) and frequencies (<10 kHz). The feasibility of moving any liquid can be predicted empirically from its frequency-dependent complex permittivity, epsilon*. The threshold for droplet actuation in air with our two-plate device configuration is /epsilon*/>8x10(-11). The mechanistic implications of these results are discussed, along with the greatly expanded range of applications for digital microfluidics that these results suggest are now feasible.
Deuterium solid-state NMR and temperature-modulated differential scanning calorimetry were used to probe the dynamics of the plasticizer di(propylene glycol) dibenzoate (DPGDB-d 10) in mixtures with poly(vinyl acetate) (PVAc). The plasticizer, deuterated in the phenyl rings, was synthesized, and 2H NMR spectra were obtained from PVAc samples with 10, 22, 27, and 37% deuterated plasticizer content as a function of temperature. The dynamics of the plasticizer in the plasticized polymer system were found to be heterogeneous with respect to different plasticizer molecules undergoing different motions. The experimental 2H NMR line shapes were fitted using a set of simulated spectra obtained from the MXQET program. The simulations were based on the superposition of two types of motion: a two-site jump motion, i.e., 180° ring flips, plus isotropic motions. The presence of the polymer allowed more plasticizer molecules to undergo 180° ring flips than in the bulk plasticizer. For the average of the log of the rate constants for the ring flips (⟨log k⟩) versus 1/temperature was linear with an apparent energy of activation of 75 kJ/mol for ring flips. From both NMR and TMDSC, the reduction in T g was proportional to the amount of plasticizer added. In addition, the T gs of DPGDB-d 10/PVAc as a function of plasticizer content were found to be similar to those of PVAc-d 3/DPGDB as determined by NMR The NMR data for both the polymer and plasticizer and TMDSC data may be fit to a plasticization model of Jenkel and Heusch with an interaction parameter b = −0.53, suggesting that both species were sensitive to the same local environment.
Summary The behavior of an amorphous polymer, poly(methyl methacrylate) (PMMA), adsorbed on silica was studied using temperature‐modulated differential scanning calorimetry (TMDSC). A two‐component model, based on loosely‐bound polymer with a glass transition temperature (Tg) (similar to that of the bulk polymer) and a tightly‐bound polymer (with a Tg higher than that of the loosely‐bound polymer) was used to interpret the thermograms. Increased sensitivity allowed the two transitions in the thermograms to be quantified much more accurately than in previous work. Linear regression analysis of the ratio of the area under two transitions with composition yielded the amount of tightly bound polymer, m″B = 1.21 +/− 0.21 mg PMMA/m2silica. Two methods of analyzing the thermograms, fitting with a Gaussian‐Lorentzian (GL) cross distribution function and perpendicular drop (PD) method, yielded similar results for the amount of tightly‐bound polymer on the surfaces with the GL method having a statistically better fit to the model. The ratio of heat capacity increments of loosely bound and tightly bound polymer, ΔCpA/ΔCpB, around the glass transition, indicated the relative mobility of the two components. It was found that the ΔCpA was aboutthree times as large as that of ΔCpB suggesting that the tightly bound polymer had a much smaller change in mobility through glass transition region.
When only tightly bound poly(vinyl acetate) (PVAc) was adsorbed on silica, the plasticizer, di(propylene glycol) dibenzoate (DPGDB), was not effective in lowering its glass transition. To determine why the plasticizer was not effective, we probed the behavior of the deuterated plasticizer di(propylene glycol) dibenzoate (DPGDB-d 10)/PVAc/silica system using deuterium (2H) solid state NMR and temperature-modulated differential scanning calorimetry (TMDSC). PVAc with 37% (w/w) of plasticizer/polymer was adsorbed on silica in adsorbed amounts of 2.60 and 0.76 mg PVAc/m2 silica. The dynamics of the plasticizer in the adsorbed PVAc was found to be more motionally heterogeneous than that observed in bulk samples. The NMR results provided firm evidence that when there is only a small amount of adsorbed polymer (e.g., only tightly bound polymer at less than 0.8 mg/m2), the plasticizer was effectively excluded from the polymer chains at the silica–polymer–air interface. When excluded from the tightly bound polymer, the plasticizer existed in an environment that was similar to that of pure plasticizer. At larger adsorbed amounts, the plasticizer was effective at lowering the T g of the adsorbed polymer.
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