We experimentally study the production of micrometer-sized droplets using microfluidic technology and a flow-focusing geometry. Two distinct methods of flow control are compared: (i) control of the flow rates of the two phases and (ii) control of the inlet pressures of the two phases. In each type of experiment, the drop size l, velocity U and production frequency f are measured and compared as either functions of the flow-rate ratio or the inlet pressure ratio. The minimum drop size in each experiment is on the order of the flow focusing contraction width a. The variation in drop size as the flow control parameters are varied is significantly different between the flow-rate and inlet pressure controlled experiments.

This manuscript describes an experimental study of the production of micro-scale droplets of the room-temperature liquid alloy eutectic gallium indium (EGaIn) formed using a microfluidic flow-focusing device. The EGaIn surface oxidizes readily to form a passivating oxide "skin" that imparts some mechanical stability to the resulting microspheres, but does not appear to affect the dynamics of droplet formation. EGaIn has an interfacial tension nearly an order of magnitude larger than typical water-in-oil systems that are used to study droplet production in microfluidic flow-focusing devices. The size of the microdroplets increase as the ratio of the flow rates of the dispersed and continuous-phase increase for both EGaIn-in-glycerol and water-in-oil systems; however, these fluid pairs form droplets through different dispersing modes at otherwise identical flow conditions (i.e., flow rate ratios and capillary numbers). Consequently, the EGaIn droplets are larger than the water droplets. The difference in dispersing modes and droplet size are attributed to the relatively larger interfacial and inertial forces of the EGaIn system compared to the water-in-oil system. The addition of polyvinyl alcohol (PVA), which is known to bind to oxide surfaces, to the continuous phase yields stable, monodisperse emulsions of liquid metal. These emulsions can be destabilized on demand by changing the solution pH, allowing the liquid metal to be recovered. The ability of the PVA to bind to the liquid metal also influences droplet production by changing the shape of the liquid as it approaches the orifice of the flow focusing device, which results in droplets with smaller diameters relative to those formed without PVA.

The adsorption of sevens-triazines from aqueous solutions by organic soil colloids was determined at pH levels from 1.0 to 5.2. Maximum adsorption occurred at pH levels in the vicinity of the pKAvalues of the respective compounds. The amounts adsorbed were dependent upon the molecular structures of the compounds and the pH of the suspension and were, in order of decreasing adsorption, as follows: 2-methoxy-4,6-bis(diethylamino)-s-triazine (hereafter referred to as tetraetatone) = 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne) = 2-hydroxy-4,6-bis(isopropylamino)-s-triazine (hereafter referred to as hydroxypropazine) > 2-methoxy-4-diethylamino-6-ethylamino-s-triazine (hereafter referred to as trietatone) > 2-methoxy-4,6-bis(isopropylamino)-s-triazine (prometone) > 2-methoxy-4,6-bis(ethylamino-s-triazine (simetone) > 2-chloro-4,6-bis(isopropylamino-s-triazine (propazine). Approximately 52% of the prometone adsorbed by the organic matter was desorbed with two extractions of 0.1N NaCl. It was concluded that the adsorption of thes-triazines was due to complexing of the triazine molecules with functional groups on the organic colloids and/or adsorption ofs-triazine cations by ion exchange forces.

The flow of viscous, particle-laden wetting thin films on an inclined plane is studied experimentally as the particle concentration is increased to the maximum packing limit. The slurry is a non-neutrally buoyant mixture of silicone oil and either solid glass beads or glass bubbles. At low concentrations ͑ Ͻ 0.45͒, the elapsed time versus average front position scales with the exponent predicted by Huppert ͓Nature ͑London͒ 300, 427 ͑1982͔͒. At higher concentrations, the average front position still scales with the exponent predicted by Huppert on some time interval, but there are observable deviations due to internal motion of the particles. At the larger concentration values and at later times, the departure from Huppert is seen to strongly depend on total slurry volume V T , inclination angle ␣, density difference, and particle size range.

Microfluidic flow-focusing technology is used to investigate the effect on drop formation due to the production of a surfactant via an interfacial chemical reaction. The reactants are an aqueous solution of sodium hydroxide (NaOH) and a mixture of oleic acid (C(17)H(33)-COOH) and mineral oil, for the dispersed and continuous phase fluids, respectively, at concentration < or = 5 mM. In the absence of a chemical reaction, the drop shapes remain constant from just after breakup into droplets down at the flow-focusing nozzle until the drops exit the channel. In the presence of the chemical reaction, there is modification of the shape depending on the concentration of reactants. The drop speeds, O(10) mm/s, lengths, O(1-100) microm, and relative displacements, O(100-1000) microm, are measured for a variety of flow conditions with observable trends that correlate with the reaction rate, which we rationalize by using the Damkohler number to characterize drop production and transport in these types of flows.