DL 11 Microflow Control using Reversible Gel Formation

DL 11 Microflow Control using Reversible Gel Formation Boris Stoeber 1, Dorian Liepmann 2, Susan J. Muller 1 1 Department of Chemical Engineering, 2 Department of Bioengineering, University of California, Berkeley Active and passive microflow-management based on heat-induced gel formation of triblock copolymer solutions. Introduction Reliable microvalves are crucial to flow control in microfluidic devices. While many types of mechanical microvalves require elaborate fabrication processes that make integration into complex systems nontrivial, the valves often suffer from slow actuation, high leakage rates or failure of moving parts. In an alternative flow control strategy, poloxamers, which are biodegradable, thermally responsive triblock copolymers, can simply be mixed into the working fluid for active and passive microflow management. The solution gels reversibly at elevated temperatures so that a microchannel can be completely and reliably blocked using integrated resistive heaters for localized heating. The simple fabrication process of the resistive heaters allows easy integration into a complex device. Further, we demonstrate that viscous heating within the fluid can lead to flowinduced gel formation, if the fluid temperature is close to the gel point. This effect can be useful for automatic flow control, where the flow rate is modulated in response to environmental temperature changes. © 2004 University of California Prepublication Data Fall 2004

Poloxamer Solutions Poloxamer triblock copolymers (PEOx-PPOy-PEOx) with a hydrophobic poly propylene oxide central block and hydrophilic poly ethylene oxide side chains are available from BASF under the registered tradename Pluronic®. They are commonly used as surfactants, emulsifiers, solubilizers and stabilizers. Poloxamers aggregate in aqueous solutions into micelles that form soft cubic crystal gel structures at elevated temperatures and concentrations. Active Poloxamer Valve Microchannels (height: 10 µm) with integrated polysilicon heaters (R= 1. 5 kΩ) © 2004 University of California Prepublication Data Fall 2004

Visualization of Valve Actuation Digital particle image velocimetry (DPIV) has been performed using an epifluorescent microscope, 0. 7 µm large fluorescent tracer particles, a digital camera and the software package Pixel. Flow 2. 1 by General Pixels. Images are captured at a frame rate of 30 Hz. Cross-correlating the intensity field of corresponding regions of consecutive images yields the displacement field and thus the velocity field of the flow. The heater in the lower branch of the channel system is actuated at 100 ms long 400 m. W pulses. The valve closes within less than 33 ms (above) upon activating, and it opens again within less than 33 ms (below). The fluid contained Pluronic® F 127 at 15 wt% at an ambient temperature of 15°C. The vector length corresponds to the local velocity. Flow Velocity [µm/s] 0 40 80 Closing time < 33 ms Opening time < 33 ms 120 © 2004 University of California Prepublication Data Fall 2004

Passive Flow Control using Viscous Heating At ambient temperatures close to the gel formation temperature, flowinduced viscous heating can lead to gel formation in a microchannel. The viscous dissipation of kinetic energy per unit volume for channel flow depends on the viscosity μ and on the strain rate tensor where v is the fluid velocity. For flow through a pipe of diameter D the wall strain rate is proportional to the volumetric flow rate q. Self-Regulating Valve At a constant volumetric flow rate q the rate of viscous dissipation becomes Thus, if viscous heating leads to gel formation on the channel walls, this reduces the channel diameter, while simultaneously the viscosity of the liquid poloxamer solution increases simply because of the heating. This leads to more viscous heating until the entire channel is blocked. This effect has been verified experimentally with a 13 wt% Pluronic® F 127 solution at a constant volumetric flow rate q = 0. 5 µl/min through a 150 µm wide, 100 µm deep and 10 mm long channel. The pressure drop along the channel (upper right) initially reaches a constant value before increasing first slowly and then more rapidly when the channel is blocked. Holding ice against the device results in a rapid pressure decrease, which confirms this assumption of shear-induced gel formation. © 2004 University of California The same behavior has been observed at different ambient temperatures, where gel formation occurs faster at higher temperatures (bottom right). Prepublication Data Fall 2004

Channel Flow Instabilities 18. 1 wt% Pluronic F 127, T = 24°C At a constant pressure drop Δp along the flow channel the volumetric flow rate becomes Flow direction so that the rate of viscous dissipation depends on the diameter D and the viscosity μ in a significantly different way than previously. If viscous heating leads to gel formation on the channel walls in the case of a constant pressure drop along a flow channel, this initially reduces the channel diameter, while simultaneously the viscosity of the liquid poloxamer solution increases as before. However, both these effects lead to reduced viscous heating, which allows for cooling of the fluid due to heat conduction into the substrate. It is therefore possible that this mechanism leads to flow instabilities. A 18. 1 wt% Pluronic F 127 solution has been seeded with 0. 7 µm large fluorescent polystyrene beads for flow visualization. The fluid is driven through a microchannel with a cross-section of 100 µm x 100 µm with a constant pressure at ambient temperature T = 24°C. The video frames (right) show that the flow partially gels periodically on the channel wall and becomes liquid again. © 2004 University of California 2. 2 s Prepublication Data Fall 2004

Multi-Stage Micromixer Objective: Stretching of the interface between the fluids to be mixed by folding the fluid layers in order to allow diffusion to act more efficiently. steps 2 This mixer concept can be realized with stages flow any other active valves than poloxamer 3 valves. stages 2 -1, (1 choppi 2) ng 2 -stage Mixer 4 -1, (23) % layer inversion 3 -4 -2, (1 -2 -4) pulsatile merging 2 -1, (1 -2) 1 -2, (3 -4) 3 -stage Mixer Cycling though the 4 working steps of the micromixer results in mixing of 2 different fluids in 3 stages. © 2004 University of California Prepublication Data Fall 2004