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Thermal Modulation and Instability of Viscous Microjets

E.P. Furlani
Eastman Kodak Company, US

microjet instability, marangoni effect, surface tension, thermal modulation

Liquid microjets are inherently unstable and can be broken into droplets by various means including the modulation of pressure, velocity, and/or fluid properties. In this presentation we discuss the controlled breakup of viscous microjets via thermal modulation of surface tension. Such modulation can be implemented using CMOS/MEMS technology by integrating resistive heating elements into the orifice manifold surrounding each orifice, as depicted in Fig. 1. When the heating elements are pulsed in a time-wise periodic fashion, the thermal energy they produce is imparted to the surface of the microjet as it leaves the orifice. This energy is carried downstream by the jet velocity, and produces a time-dependent spatially periodic variation of surface tension along the jet that causes breakup and drop formation (Figs. 1 and 2). Using this method, microfluidic devices can be fabricated with thousands of functional microjets, each of which can be individually modulated to produce steady steams of picoliter-sized droplets. Moreover, these devices can operate at kilohertz frequency rates achieving unprecedented speed and versatility of droplet generation for applications such as inkjet prining.1, 2 In this presentation, we discuss theoretical models for predicting the instability of viscous microjets that are subjected to a thermally induced modulation of surface tension. We present analytical formulas for predicting the free-surface, velocity, and pressure within the jet. We use these to understand the instability physics, and to estimate the time-to-breakup Tb as a function of the modulation wavelength, fluid properties, and system parameters (Fig. 3). We also present numerical models for more accurate predictions of pinch-off and satellite formation.

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