Zachary Mills

Mesoscale modeling: solving complex flows in biology and biotechnology

Fluids are involved in practically all physiological activities of living organisms. However, biological and biorelated flows are hard to analyze due to the inherent combination of interdependent effects and processes that occur on a multitude of spatial and temporal scales. Recent advances in mesoscale simulations enable researchers to tackle problems that are central for the understanding of such flows. Furthermore, computational modeling effectively facilitates the development of novel therapeutic approaches. Among other methods, dissipative particle dynamics and the lattice Boltzmann method have become increasingly popular during recent years due to their ability to solve a large variety of problems. In this review, we discuss recent applications of these mesoscale methods to several fluid-related problems in medicine, bioengineering, and biotechnology.

Wet mammals shake at tuned frequencies to dry

In cold wet weather, mammals face hypothermia if they cannot dry themselves. By rapidly oscillating their bodies, through a process similar to shivering, furry mammals can dry themselves within seconds. We use high-speed videography and fur particle tracking to characterize the shakes of 33 animals (16 animals species and five dog breeds), ranging over four orders of magnitude in mass from mice to bears. We here report the power law relationship between shaking frequency f and body mass M to be f ∼ M−0.22, which is close to our prediction of f ∼ M−0.19 based upon the balance of centrifugal and capillary forces. We also observe a novel role for loose mammalian dermal tissue: by whipping around the body, it increases the speed of drops leaving the animal and the ensuing dryness relative to tight dermal tissue.

Beating synthetic cilia enhance heat transport in microfluidic channels

Using computational modeling, we probed the utility of actuated synthetic cilia for enhancing heat transport in microfluidic channels. Cilia are elastic filaments that are attached to the bottom channel wall with a constant tilt and actuated by a periodical vertical force applied to their free ends. We show that periodical oscillations of elastic cilia mix the heated fluid and create secondary flows in the microchannel that facilitate heat transport between channel walls. The magnitude of the secondary flows and the cilium deformation pattern are controlled by the frequency and amplitude of the periodic driving force. Thus, by varying the force parameters one can effectively regulate the local heat transport in ciliated microchannels.

Onset of unsteady flow in wavy walled channels at low Reynolds number

Using computational modeling, we examine the development of an unsteady laminar flow of a Newtonian fluid in a channel with sinusoidal walls. The flow is driven by a constant pressure gradient. The simulations reveal two types of unsteady flows occurring in sinusoidal channels. When the amplitude of the wavy walls is relatively small, vortices forming in the channel furrows are shed downstream. For larger wall wave amplitudes, vortices remain inside the furrows and exhibit periodic oscillations and topological changes. We present a phase diagram in terms of wall amplitude and driving pressure gradient separating different flow regimes. Our simulations establish the optimum wall amplitude and period leading to an unsteady flow at the minimum pressure gradient. The results are important for designing laminar heat/mass exchangers utilizing unsteady flows for enhancing transport processes.

Active fluid mixing with magnetic microactuators for capture of salmonella

Detection of low concentrations of bacteria in food samples is a challenging process. Key to this process is the separation of the target from the food matrix. We demonstrate magnetic beads and magnetic micro-cilia based microfluidic mixing and capture, which are particularly useful for pre-concentrating the target. The first method we demonstrate makes use of magnetic microbeads held on to NiFe discs on the surface of the substrate. These beads are rotated around the magnetic discs by rotating the external magnetic field. The second method we demonstrate shows the use of cilia which extends into the fluid and is manipulated by a rotating external field. Magnetic micro-features were fabricated by evaporating NiFe alloy at room temperature, on to patterned photoresist. The high magnetic permeability of NiFe allows for maximum magnetic force on the features. The magnetic features were actuated using an external rotating magnet up to frequencies of 50Hz. We demonstrate active mixing produced by the microbeads and the cilia in a microchannel. Also, we demonstrate the capture of target species in a sample using microbeads.