Siva A. Vanapalli

Stability of emulsions to dispersed phase crystallization: effect of oil type, dispersed phase volume fraction, and cooling rate

Abstract Emulsified n -hexadecane or confectionery coating fat (CCF) were repeatedly thermally cycled (40 to −10–40xa0°C at 1.5xa0°C min −1 ) in a differential scanning calorimeter. There is a single exotherm on cooling each emulsion (at 0.5 and 5xa0°C, respectively) corresponding to lipid crystallization by homogeneous nucleation. However, on subsequent cooling cycles, an increasing proportion of the CCF crystallization enthalpy occurs at a higher temperature (15xa0°C) corresponding to crystallization of non-emulsified fat. (The net enthalpy of crystallization is constant). The second peak is taken to be due to destabilized fat and increases with number of cycles and dispersed phase volume fraction. All n -hexadecane emulsions are stable to freeze-thaw. When the CCF emulsions are cooled more rapidly (5xa0°C min −1 ) there is no destabilization. Micrographs are presented and used to argue the destabilization is due to a collapse of partially coalesced structures on reheating.

Microfluidics as a functional tool for cell mechanics

Living cells are a fascinating demonstration of natures most intricate and well-coordinated micromechanical objects. They crawl, spread, contract, and relax-thus performing a multitude of complex mechanical functions. Alternatively, they also respond to physical and chemical cues that lead to remodeling of the cytoskeleton. To understand this intricate coupling between mechanical properties, mechanical function and force-induced biochemical signaling requires tools that are capable of both controlling and manipulating the cell microenvironment and measuring the resulting mechanical response. In this review, the power of microfluidics as a functional tool for research in cell mechanics is highlighted. In particular, current literature is discussed to show that microfluidics powered by soft lithographic techniques offers the following capabilities that are of significance for understanding the mechanical behavior of cells: (i) Microfluidics enables the creation of in vitro models of physiological environments in which cell mechanics can be probed. (ii) Microfluidics is an excellent means to deliver physical cues that affect cell mechanics, such as cell shape, fluid flow, substrate topography, and stiffness. (iii) Microfluidics can also expose cells to chemical cues, such as growth factors and drugs, which alter their mechanical behavior. Moreover, these chemical cues can be delivered either at the whole cell or subcellular level. (iv) Microfluidic devices offer the possibility of measuring the intrinsic mechanical properties of cells in a high throughput fashion. (v) Finally, microfluidic methods provide exquisite control over drop size, generation, and manipulation. As a result, droplets are being increasingly used to control the physicochemical environment of cells and as biomimetic analogs of living cells. These powerful attributes of microfluidics should further stimulate novel means of investigating the link between physicochemical cues and the biomechanical response of cells. Insights from such studies will have implications in areas such as drug delivery, medicine, tissue engineering, and biomedical diagnostics.

Emulsions under shear: the formation and properties of partially coalesced lipid structures

Partial coalescence in semi-crystalline emulsion droplets is essential to the manufacture and quality of many dairy products including ice cream, whipped cream, and butter. In real food emulsions this process occurs under fluctuating temperatures and shear rates and some of the effects of these factors are considered in this work. Two approaches to the problem are reported. In the first, the freeze-thaw stability and shear modulus of a food oil emulsion during a freeze-thaw cycle is related to the volume fraction of dispersed phase. In the second approach the rate of crystallization of emulsified n-hexadecane in the presence of solid n-hexadecane is shown to be independent of applied shear rate. Some ideas on how these approaches may be combined for a fuller understanding of the partial coalescence process are presented. La coalescence partielle dans les gouttelettes demulsion semicristalline est essentielle a la fabrication et a la qualite de nombreux produits laitiers tels que glace, creme fouettee et beurre. Dans des emulsions alimentaires ce processus se produit a des temperatures et a des taux de cisaillement variables et les effets de ces facteurs sont examines dans cette etude.

Universal scaling for polymer chain scission in turbulence

We report that previous polymer chain scission experiments in strong flows, long analyzed according to accepted laminar flow scission theories, were in fact affected by turbulence. We reconcile existing anomalies between theory and experiment with the hypothesis that the local stress at the Kolmogorov scale generates the molecular tension leading to polymer covalent bond breakage. The hypothesis yields a universal scaling for polymer scission in turbulent flows. This surprising reassessment of over 40 years of experimental data simplifies the theoretical picture of polymer dynamics leading to scission and allows control of scission in commercial polymers and genomic DNA.

Growth kinetics of microalgae in microfluidic static droplet arrays.

We investigated growth kinetics of microalgae, Chlorella vulgaris, in immobilized arrays of nanoliter‐scale microfluidic drops. These static drop arrays enabled simultaneous monitoring of growth of single as well as multiple cells encapsulated in individual droplets. To monitor the growth, individual drop volumes were kept nearly intact for more than a month by controlling the permeation of water in and out of the microfluidic device. The kinetic growth parameters were quantified by counting the increase in the number of cells in each drop over time. In addition to determining the kinetic parameters, the cell‐size distribution of the microalgae was correlated with different stages of the growth. The single‐cell growth kinetics of C. vulgaris showed significant heterogeneity. The specific growth rate ranged from 0.55 to 1.52u2009day−1 for different single cells grown in the same microfluidic device. In comparison, the specific growth rate in bulk‐scale experiment was 1.12u2009day−1. It was found that the average cell size changes significantly at different stages of the cell growth. The mean cell‐size increased from 5.99u2009±u20091.08 to 7.33u2009±u20091.3u2009µm from exponential to stationary growth phase. In particular, when multiple cells are grown in individual drops, we find that in the stationary growth phase, the cell size increases with the age of cell suggesting enhanced accumulation of fatty acids in older cells. Biotechnol. Bioeng. 2012; 109: 2987–2996.

Programmable fluidic production of microparticles with configurable anisotropy

We report a technique for continuous production of microparticles of variable size with new forms of anisotropy including alternating bond angles, configurable patchiness, and uniform roughness. The sequence and shape of the anisotropic particles are configured by exploiting a combination of confinement effects and microfluidics to pack precursor colloids with different properties into a narrow, terminal channel. The width and length of the channel relative to the particle size fully specify the configuration of the anisotropic particle that will be produced. The precursor spheres packed in the production zone are then permanently bonded into particles by thermal fusing. The flow in the production zone is reversed to release the particles for collection and use. Particles produced have linear chain structure with precisely configured, repeatable bond angles. With software programmable microfluidics, sequence and shape anisotropy are combined to yield synthesized homogeneous (type A), surfactantlike (type A-B) or triblock (type A-B-A) internal sequences in a single device. By controlling the dimensions of the microfluidic production zone, triangular prisms and particles with controlled roughness and patchiness are produced. The fabrication method is performed with precursors spheres with diameter as small as 3.0 microm.

Behavior of a train of droplets in a fluidic network with hydrodynamic traps

The behavior of a droplet train in a microfluidic network with hydrodynamic traps in which the hydrodynamic resistive properties of the network are varied is investigated. The flow resistance of the network and the individual droplets guide the movement of droplets in the network. In general, the flow behavior transitions from the droplets being immobilized in the hydrodynamic traps at low flow rates to breaking up and squeezing of the droplets at higher flow rates. A state diagram characterizing these dynamics is presented. A simple hydrodynamic circuit model that treats droplets as fluidic resistors is discussed, which predicts the experimentally observed flow rates for droplet trapping in the network. This study should enable the rational design of microfuidic devices for passive storage of nanoliter-scale drops.

Electrowetting-controlled droplet generation in a microfluidic flow-focusing device

We studied the generation of aqueous microdrops in an oil–water flow-focusing device with integrated insulator-covered electrodes that allow for continuous tuning of the water wettability by means of electrowetting. Depending on the oil and water inlet pressures three different operating conditions were identified that shift upon applying a voltage: stable oil–water interface, drop generation, and laminar water jet formation. Full control over the drop generation is achieved within a well-defined range of inlet pressures, in quantitative agreement with a model based on the additive contributions from electrowetting and the local hydrostatic pressure at the junction. The tuning power of electrowetting is shown to increase upon device miniaturization, which makes this approach particularly attractive for flow control on the sub-micrometer scale.

Scission-induced bounds on maximum polymer drag reduction in turbulent flow

We report the direct quantification of molar mass degradation in the drag-reducing polymers polyethylene oxide (PEO) and polyacrylamide (PAM) in turbulent pipe flows with an upstream tapered contraction. We find that entrance effects associated with the upstream contraction dominate the polymer degradation. Quantifying degradation according to the scaling relationship γw∝Mws−n, the exponent n is determined to be −2.20±0.21 and −2.73±0.18 for PEO and PAM, respectively. Here Mws is the steady-state (or limiting) weight-average scission molar mass. A methodology is devised to circumvent polymer degradation due to the upstream contraction and thereby conduct degradation experiments in which only the turbulent flow in the pipe is responsible for chain scission. In this case, the scission-scaling relationship for PEO is γw∝Mw−3.20±0.28. Here Mw is the degraded weight-average molar mass after one pass through the 1.63-m length of pipe. Based on these scaling relationships we obtain a new upper limit for polyme...

Electrowetting --A versatile tool for controlling microdrop generation

Abstract.Integrating insulator-covered electrodes into a microfluidic flow focusing device (FFD) we demonstrate enhanced flexibility and control of the flow of two non-miscible liquids based on electrowetting (EW). In the parameters space, determined by liquid inlet pressures, we identify a specific region where drops can only be generated and addressed via EW. In this regime we show that the size distribution and the frequency of drop generation can be controlled by the applied voltage and the width of voltage pulses. Moreover it turns out that with EW the drop size and the frequency can be tuned independently. Finally we show that the same drop generation phenomena can also be observed in the presence of surfactants.