Lothar Schmid

Controlled surface-induced flows from the motion of self-assembled colloidal walkers

Biological flows at the microscopic scale are important for the transport of nutrients, locomotion, and differentiation. Here, we present a unique approach for creating controlled, surface-induced flows inspired by a ubiquitous biological system, cilia. Our design is based on a collection of self-assembled colloidal rotors that “walk” along surfaces in the presence of a rotating magnetic field. These rotors are held together solely by magnetic forces that allow for reversible assembly and disassembly of the chains. Furthermore, rotation of the magnetic field allows for straightforward manipulation of the shape and motion of these chains. This system offers a simple and versatile approach for designing microfluidic devices as well as for studying fundamental questions in cooperative-driven motion and transport at the microscopic level.

Particle deflection in a poly(dimethylsiloxane) microchannel using a propagating surface acoustic wave: size and frequency dependence.

We study the effect of a propagating surface acoustic wave (PSAW) with different frequencies on particles with different sizes in microfluidic channels. We find that the deflection critically depends on the applied frequency as well as on the particle size. For fixed frequencies, large particles are deflected and migrate perpendicular to the flow direction while smaller particles only follow the streamlines of the flow field. However, with increasing frequency of the PSAW above a size dependent limit, small particles are also actuated. This relation can be characterized by the wavenumber k and the particle radius r using the parameter κ = k · r. For the onset of deflection, we find a critical value κc ≅ 1.28 ± 0.20. Finally, we demonstrate how this device can be used for particle separation.

Separation of blood cells using hydrodynamic lift

Using size and deformability as intrinsic biomarkers, we separate red blood cells (RBCs) from other blood components based on a repulsive hydrodynamic cell-wall-interaction. We exploit this purely viscous lift effect at low Reynolds numbers to induce a lateral migration of soft objects perpendicular to the streamlines of the fluid, which closely follows theoretical prediction by Olla [J. Phys. II 7, 1533, (1997)]. We study the effects of flow rate and fluid viscosity on the separation efficiency and demonstrate the separation of RBCs, blood platelets, and solid microspheres from each other. The method can be used for continuous and label-free cell classification and sorting in on-chip blood analysis.

Magneto-mechanical mixing and manipulation of picoliter volumes in vesicles

Superparamagnetic beads in giant unilamellar vesicles are used to facilitate magnetic manipulation, positioning, agitation and mixing of ultrasmall liquid volumes. Vesicles act as leakproof picoliter reaction vessels in an aqueous bulk solution and can be deliberately conveyed by an external magnetic field to a designated position. Upon application of an external magnetic field the beads align to form extended chains. In a rotating magnetic field chains break up into smaller fragments caused by the interplay of viscous friction and magnetic attraction. This process obeys a simple relationship and can be exploited to enhance mixing of the vesicle content and the outer solution or adjacent vesicle volumes exactly at the position of release.

Acoustic modulation of droplet size in a T-junction

We introduce an approach and describe the process of acoustically driven formation of droplets in a microfluidic T-junction. Our system allows for fast and precise control of drop volume over a wide range that is fully electrically triggered. We exploit the interaction of a surface acoustic wave (SAW) excited on a piezoelectric, transparent substrate with the fluid to adjust the size of drops in a continuous microflow in real time and relate SAW intensity and drop size. Our device operates in the squeezing regime at low capillary numbers. We describe the mechanism of SAW modulated formation of a monodisperse microemulsion that forms the basis for the integration of more complex operations useful for droplet fluidic systems.

Dynamics of fluid vesicles in flow through structured microchannels

The dynamics of fluid vesicles is studied under flow in microchannels, in which the width varies periodically along the channel. Three types of flow instabilities of prolate vesicles are found. For small quasi-spherical vesicles —compared to the average channel width— perturbation theory predicts a transition from a state with orientational oscillations of a fixed prolate shape to a state with shape oscillations of symmetrical ellipsoidal or bullet-like shapes with increasing flow velocity. Experimentally, such orientational oscillations are observed during the slow migration of a vesicle towards the centerline of the channel. For larger vesicles, mesoscale hydrodynamics simulations and experiments show similar symmetric shape oscillation at reduced volumes V*0.9. However, for non-spherical vesicles with V*0.9, shapes are found with two symmetric or a single asymmetric tail.

Hydrodynamic deformation reveals two coupled modes/time scales of red blood cell relaxation

We study the mechanical relaxation behavior of human red blood cells by observing the time evolution of shape change of cells flowing through microchannels with abrupt constrictions. We observe two types of relaxation processes. In the first fast process (τ1 ∼ 200 ms) the initially parachute shaped cells relax into cup-shaped cells (stomatocytes). These cells relax and reorient in a second relaxation process with a response time of τ1/2 ∼ 10 s into the equilibrium discoid shapes. The values for the relaxation times of single red blood cells in the population scatter significantly within the cell population between 0.11 s < τ1 < 0.52 s and 9 s < τ1/2 < 49 s, respectively. However, when plotting τ1/2 against τ1, we find a linear relationship between the two timescales and are able to relate both to the elastic properties of the spectrin cytoskeleton underlying the red cells plasma membrane. Adenosine Triphosphate (ATP) enhances dissociation of spectrin filaments resulting in a reduced shear modulus. We modify the cytoskeleton connectivity by depletion and repletion of ATP and study the effect on relaxation. Both the linear relationship of timescales as well as the ATP dependence can be understood by theoretical models.

Dynamics of red blood cells and vesicles in microchannels of oscillating width.

We have studied the dynamics of red blood cells and fluid lipid vesicles in hydrodynamic flow fields created by microchannels with periodically varying channel width. For red blood cells we find a transition from a regime with oscillating tilt angle and fixed shape to a regime with oscillating shape with increasing flow velocity. We have determined the crossover to occur at a critical ratio L(y)/v(m) ≈ 2.2 × 10⁻³ s with channel width L(y) and red blood cell velocity v(m). These oscillations are superposed by shape transitions from a discocyte to a slipper shape at low velocities and a slipper to parachute transition at high flow velocities.

Chapter 16:Surface Acoustic Wave Based Microfluidics and Droplet Applications

Surface acoustic waves are used to induce acoustic streaming in small amounts of liquid on a chip surface. Both mixing as well as actuation of the fluid can be achieved in an efficient and controllable manner. This way, highly complex chip based assay laboratories can be created. Combined with elastomer microfluidic devices and droplet based microreactors, high speed and very selective cell sorters have been recently demonstrated.

Real-time size modulation and synchronization of a microfluidic dropmaker with pulsed surface acoustic waves (SAW)

We show that a microfluidic flow focusing drop maker can be synchronized to a surface acoustic waves (SAW) triggered by an external electric signal. In this way droplet rate and volume can be controlled over a wide range of values in real time. Using SAW, the drop formation rate of a regularly operating water in oil drop maker without SAW can be increased by acoustically enforcing the drop pinch-off and thereby reducing the volume. Drop makers of square cross-sections (w = h = 30 µm, with width w and height h) that produce large drops of length l = 10 w can be triggered to produce drops as short as l ~ 2w, approaching the geometical limit l = w without changing the flow rates. Unlike devices that adjust drop size by changing the flow rates the acoustic dropmaker has very short transients allowing to adjust the size of every single drop. This allows us to produce custom made emulsions with a defined size distribution as demonstrated here not only for a monodisperse emulsion but also for binary emulsions with drops of alternating size. Moreover, we show that the robustness and monodispersity of our devices is enhanced compared to purely flow driven drop makers in the absence of acoustic synchronization.