Piotr Garstecki

Formation of monodisperse bubbles in a microfluidic flow-focusing device

This letter describes a method for generating monodisperse gaseous bubbles in a microfluidic flow-focusing device. The bubbles can be obtained in a range of diameters from 10 to 1000μm. The volume Vb of the bubbles scales with the flow rate q and the viscosity μ of the liquid, and the pressure p of the gas stream as Vb∝p∕qμ. This method allows simultaneous, independent control of the size of the individual bubbles and volume fraction of the dispersed phase. Under appropriate conditions, bubbles self-assemble into highly ordered, flowing lattices. Structures of these lattices can be adjusted dynamically by changing the flow parameters.

Transition from squeezing to dripping in a microfluidic T-shaped junction

We describe the results of a numerical investigation of the dynamics of breakup of streams of immiscible fluids in the confined geometry of a microfluidic T-junction. We identify three distinct regimes of formation of droplets: squeezing, dripping and jetting, providing a unifying picture of emulsification processes typical for microfluidic systems. The squeezing mechanism of breakup is particular to microfluidic systems, since the physical confinement of the fluids has pronounced effects on the interfacial dynamics. In this regime, the breakup process is driven chiefly by the buildup of pressure upstream of an emerging droplet and both the dynamics of breakup and the scaling of the sizes of droplets are influenced only very weakly by the value of the capillary number. The dripping regime, while apparently homologous to the unbounded case, is also significantly influenced by the constrained geometry; these effects modify the scaling law for the size of the droplets derived from the balance of interfacial and viscous stresses. Finally, the jetting regime sets in only at very high flow rates, or with low interfacial tension, i.e. higher values of the capillary number, similar to the unbounded case.

Escherichia coli swim on the right-hand side.

The motion of peritrichously flagellated bacteria close to surfaces is relevant to understanding the early stages of biofilm formation and of pathogenic infection. This motion differs from the random-walk trajectories of cells in free solution. Individual Escherichia coli cells swim in clockwise, circular trajectories near planar glass surfaces. On a semi-solid agar substrate, cells differentiate into an elongated, hyperflagellated phenotype and migrate cooperatively over the surface, a phenomenon called swarming. We have developed a technique for observing isolated E. coli swarmer cells moving on an agar substrate and confined in shallow, oxidized poly(dimethylsiloxane) (PDMS) microchannels. Here we show that cells in these microchannels preferentially ‘drive on the right’, swimming preferentially along the right wall of the microchannel (viewed from behind the moving cell, with the agar on the bottom). We propose that when cells are confined between two interfaces—one an agar gel and the second PDMS—they swim closer to the agar surface than to the PDMS surface (and for much longer periods of time), leading to the preferential movement on the right of the microchannel. Thus, the choice of materials guides the motion of cells in microchannels.

Mixing with bubbles: a practical technology for use with portable microfluidic devices

This paper demonstrates a methodology for micromixing that is sufficiently simple that it can be used in portable microfluidic devices. It illustrates the use of the micromixer by incorporating it into an elementary, portable microfluidic system that includes sample introduction, sample filtration, and valving. This system has the following characteristics: (i) it is powered with a single hand-operated source of vacuum, (ii) it allows samples to be loaded easily by depositing them into prefabricated wells, (iii) the samples are filtered in situ to prevent clogging of the microchannels, (iv) the structure of the channels ensure mixing of the laminar streams by interaction with bubbles of gas introduced into the channels, (v) the device is prepared in a single-step soft-lithographic process, and (vi) the device can be prepared to be resistant to the adsorption of proteins, and can be used with or without surface-active agents.

Simultaneous generation of droplets with different dimensions in parallel integrated microfluidic droplet generators

This paper describes geometric coupling of the dynamics of break-up of liquid threads in parallel flow-focusing devices (FFD), which are integrated into a multiple quadruple-microfluidic droplet generator (QDG). We show weak parametric coupling between parallel FFDs with an identical design, which leads to the slight broadening of the distribution of sizes of droplets. Using parallel FFDs with distinct geometries we simultaneously generated several populations of droplets with different volumes, yet, each of these populations was characterized by a narrow size distribution. Simulation of the generation of droplets in the quadruple-microfluidic droplet generator based on hydraulic resistances to the flow of a single-phase fluid was in good agreement with the experimental results.

Formation of bubbles and droplets in parallel, coupled flow-focusing geometries.

This paper describes the mechanism of formation of bubbles of nitrogen in water containing Tween 20 as a surfactant, and of droplets of water in hexadecane containing Span 80 as a surfactant. The study of these microfluidic systems compares two or four flow-focusing generators coupled through shared inlets, supplying the continuous phase, and through a common outlet channel. The processes that form bubbles in neighboring generators interact for a wide range of flow parameters; the formation of bubbles alternates in time and space, and the bubbles assemble into complex patterns in the outlet channel. The dynamics of formation of bubbles in these systems are stable for long time (at least 10 min). For a certain range of flow parameters, the coupled flow-focusing generators exhibit two stable modes of operation for a single set of flow parameters. The dynamics of formation of droplets of water in hexadecane by the coupled flow-focusing generators are simpler--the adjacent generators produce only monodisperse droplets over the entire range of flow parameters that are explored. These observations suggest that the mechanism of interaction between coupled flow-focusing generators relies on the compressibility of the dispersed phase (e.g., the gas or liquid), and on variations in pressure at the flow-focusing orifices induced by the breakup of bubbles or droplets.

Design for mixing using bubbles in branched microfluidic channels

This letter describes a method for producing chaotic transport trajectories in planar, microfluidic networks prepared by standard, single-step lithography and operated with a steady-state inflow of the fluids into the device. Gaseous slugs flowing through the network produce temporal variation of pressure distribution and lead to stretching and folding of the continuous fluid. Stabilization of the bubbles by surface-active agents is not necessary, and the method is compatible with the wide range of reactions performed in on-chip bioassays.

The structure and stability of multiple micro-droplets

Microfluidic droplet-on-demand systems allow the controllable construction of multiple droplets of previously unattainable morphologies. Guided by the diagrams of the possible topologies of double droplets we investigate in detail the vistas to control the morphology of Janus droplets. We also explore and control new morphologies of multiple Janus droplets, i.e., arbitrarily long chains of alternating immiscible segments. Theoretical calculations together with the control offered by the use of automation allow the design of both the topology and the geometry (e.g. curvatures of the interfaces) of the multiple droplets. The ability to rationally design convex–convex, convex–concave and concave–convex segments may be useful in material science, while the ability to tune the distances between the interfaces in the chains of droplets may have applications in designing artificial biochemical signalling networks.

Bacterial Growth and Adaptation in Microdroplet Chemostats

We describe herein microfluidic technology for manipulating and monitoring continuous growth of populations of bacteria. A system consisting of approximately ten input and output channels controls more than 100 microdroplet chemostats and enables the manipulation of chemical factors in each microchemostat independently over time. Herein, we characterize the dynamics of bacterial populations in microdroplet chemostats and cellular responses to a range of stable or changing antibiotic concentrations. This method allows for parallel, long-term studies of microbial ecology, physiology, evolution, and adaptation to chemical environments. The introduction of the chemostat by Leo Szilard was a milestone in the field of microbiology. Chemostats facilitate the continuous culture of bacteria, yeast, and algae by continuously replenishing a constant volume of fluid to maintain specific concentrations of cells and growth factors. Chemostats have facilitated a wide-range of studies, including microbial ecology, predator–prey dynamics, and the evolution of drug resistance. The consumption of large quantities of reagents and the significant operational challenges of traditional chemostats limit their use. Single-phase, microfluidic versions of chemostats minimize incubation volumes, and yet are limited by their complexity: the proportionality between the number of input/ output controls and the number of chemostats hamper large scale parallelization. Single-phase microfluidic systems are prone to biofilm formation, which makes them either singleuse devices or requiring additional steps to minimize cell adhesion. Droplet microfluidics offer a unique solution to creating many parallel chemostats. The earliest example of this technology in microbiology was first demonstrated by Joshua Lederberg nearly 60 years ago. In the interim, the field of microfluidics solved many of the technical challenges associated with using this approach to study microbes. Compartmentalizing cells and nutrients in microdroplets of liquid can reduce the complexity and cost of operating many parallel chemostats. Recently, bacteria have been incubated in droplets in channels over short time intervals, however sustained cell growth over hundreds of generations in a series of fully addressable microdroplets has not been possible. Herein, we describe an automated microdroplet system that transcends existing challenges and enables users to manipulate the chemical composition of droplets for longterm bacterial studies. The microfluidic system (Figure 1) performs three functions: 1) formation of microdroplets containing cells, reagents, and soluble growth factors; 2) cycling microdroplets for cell incubation and monitoring; and 3) splitting and fusing microdroplets to control the concentration of chemical factors over time. After loading the reservoirs with liquid samples, we used a source of pressure and external valves to regulate the flow of

Propulsion of flexible polymer structures in a rotating magnetic field

We demonstrate a new concept for the propulsions of abiological structures at low Reynolds numbers. The approach is based on the design of flexible, planar polymer structures with a permanent magnetic moment. In the presence of an external, uniform, rotating magnetic field these structures deform into three-dimensional shapes that have helical symmetry and translate linearly through fluids at Re between 10(-1) and 10. The mechanism for the motility of these structures involves reversible deformation that breaks their planar symmetry and generates propulsion. These elastic propellers resemble microorganisms that use rotational mechanisms based on flagella and cilia for their motility in fluids at low Re.