We present microfluidic modules (traps) that allow us to lock, shift, dose and merge micro-aliquots of liquid precisely. The precision is hard-wired into the geometry of the device: small values of the capillary number guarantee reproducibility of operation over a range of rates of flow that need not be controlled precisely. The modules can be integrated into systems that perform complicated protocols on micro-droplets while not requiring precision in forcing the flow.
The motion of soft-glassy materials (SGM) in a confined geometry is strongly impacted by surface roughness. However, the effect of the spatial distribution of the roughness remains poorly understood from a more quantitative viewpoint. Here we present a comprehensive study of concentrated emulsions flowing in microfluidic channels, one wall of which is patterned with micron-size equally spaced grooves oriented perpendicularly to the flow direction. We show that roughness-induced fluidization can be quantitatively tailored by systematically changing both the width and separation of the grooves. We find that a simple scaling law describes such fluidization as a function of the density of grooves, suggesting common scenarios for droplet trapping and release. Numerical simulations confirm these views and are used to elucidate the relation between fluidization and the rate of plastic rearrangements. Controlling the slip and flow of soft-glassy materials (SGM) at the microscale is crucial for food and pharmaceutical processing, and for micro-manufacturing [1][2][3][4]. SGM include concentrated emulsions, gels, foams, pastes, and exhibit a complex, non-linear rheology [5][6][7]: they behave like elastic solids unless a stress large enough, known as the yield stress σ Y , is applied. Above σ Y SGM flow like non-Newtonian liquids. This solidto-liquid transition and the corresponding flowing properties have been widely studied [8], but still pose a series of challenging questions, relevant both for applications [9][10][11] and for a better understanding of the statistical mechanics of SGM [12][13][14][15][16][17][18][19][20]. Recent studies [21][22][23][24][25][26][27][28][29] showed that their flow bahavior is characterized by "nonlocality" [21,22], meaning that the relation between the local stress σ and the local shear rateγ cannot be explained with a unique master curve. This non-local behaviour depends on both confinement and surface roughness [22,25,26], and it is ascribed to the presence of plastic rearrangements [21,22], i.e. topological changes in the micro-structural configurations. These take place whenever the material cannot sustain the accumulated stress, then it undergoes an irreversible deformation and releases the excess stress in the form of elastic waves. The range of such perturbation introduces a new length, named "cooperativity length" ξ [21], which is typically on the order of a few diameters of micro-structural constituents (i.e. droplets for emulsions [21,22], bubbles for foams [27,28], blobs for gels [29], etc). Although the cooperativity length becomes relevant at the jamming point of SGM [30], it has been sharply argued that ξ is fundamentally different from the characteristic legnth that describes dynamical heterogeneities involved in spontaneous fluctuations [31][32][33]. Recently, many theoretical studies have been put forward in the recent years to account for these non-local effects [12,[15][16][17]20]. One of them, the kinetic elastoplastic (KEP) model [16], explores the effects of coopera...
We present an experimental comparative study of the effect of elasticity of the continuous liquid in generation of droplets in microfluidic flow focusing devices. For a wide range of values of dynamic viscosity of the Newtonian droplet phase we compare the dynamics of the formation of droplets in three different Newtonian and non-Newtonian, viscoelastic (Boger) fluid pairs of the same (and nearly constant) shear viscosities. In both Newtonian and viscoelastic systems we find similar regimes of operation of the system: (i) dripping without satellites, (ii) dripping with single satellites, (iii) formation of multiple satellites, and (iv) jetting. We find that the elasticity of the focusing liquid stabilizes the jets facilitating formation of smaller droplets, and leads to transitions between various regimes at lower ratios of flow and at lower values of the capillary numbers in comparison to the Newtonian focusing liquids. We also show that the polydispersity of Newtonian droplets produced in non-Newtonian continuous phases depend significantly on the viscosity of the droplet phase while this dependence is minor when the continuous liquid is Newtonian. C 2013 AIP Publishing LLC. [http://dx.
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