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This research is an active collaboration between SCU and HP-Palo Alto. This research is supported by the National Science foundation. Extensions of this work have been proposed and the research will be ongoing.
Full characterization of the boiling process, heterogenous, homogeneous or combined is desired. Also, the relationship between these phenomena and important variables such as induced power (voltage and time of applied voltage), frequency of the induced power, heater surface roughness, bulk fluid pressure, pressure wave induced by the bubble and bulk temperature is investigated. The effect of the boiling process on the wear and degration of the boiling surface is being investigated.
Boiling is induced on micro resistors through an applied electric power through the micro heaters the time scale of the applied voltage is on the order of microseconds. Therefore, a micro process in the time and length scale is underway during the boiling process.
The formation of a vapor embryo might occur if a meta stable fluid and a solid phase are in contact, this process is a form of heterogeneous boiling that could happen in micro heaters (the first set of heaters. However, since the fluid could be reaching a superheating state near the heater surface, homogeneous boiling could, as well, be present in this phenomena. Homogeneous boiling occurs in a superheated fluid with bubble formation within the meta stable superheated state (the second set of figures). Boiling is a complicated phenomena since bubbles could merge and cancel each other as illustrated in figure for the heterogeneous boiling.
Current micro-boiling work with water
Recently, a series of experiments were developed aimed to understand the micro-boiling physics taking place inside TIJ chambers. The experimental set-up for observations of the micro-boiling is shown in figure below.
Micro-boiling Visualization Experimental Set up
A transparent chamber with dimensions of a few millimeters was constructed to hold the wafer containing micro-heaters having a thickness of about 40 micrometers. The chamber is first evacuated and then filled with deionized water and the heater resistor is activated using the driving circuit. The frequency, pulse duration and voltage applied to the resistor are controlled with high precision using the custom-made circuitry. This circuit has been adapted to synchronize a diode laser. The laser is used to illuminate the boiling process and it can be activated at controlled frequencies and nanosecond durations. A microscope along with a CCD camera is used to observe the boiling phenomena and record it in a computer. The data images are later processed to identify bubble location and size. Typical input signal from the circuit driver to the micro-heater is shown in the figure below.
The first two sets of micro-boiling experiments are shown in the figure below demonstrating good repeatability. Each point was obtained from at least 200 images; the variation plotted (2*standard deviations) on the curves is small. Large errors were expected at short times due to variations in the nucleation sites. The radius growth rate in the curves is approximately 29 m/s in the range 2.3 ms-2.7 ms for the heater of 45 micrometers by 41 micrometers surface area before the bubble fills the total area of the resistor. Once the bubble reaches the total area the growth rate decreases. Ongoing experiments will probe broader time ranges and power conditions to capture the range of the formation and collapse process. For Zhao et al. for a voltage of 37 V and a resistor of 110 micrometers by 100 micrometers surface area the radius growth rate is approximately 11m/s for the time range of 5-7 ms approximately. The current results are consistent factoring the smaller resistor area and the faster growth rate. It should be pointed out that the fastest growth rate exceeds the theoretical kinetic limit based on simplified interface and equilibrium thermodynamic assumptions.
Experimental results from Zaho et al. 2000
With the infrastructure in place, a set of experiments will be designed to answer fundamental questions about the micro-boiling process. This experimental set up will be used to study different fluids. Using a HP-Aguadilla laser based profilometer, we have the capability to characterized the surface prior to and after an experiment. The figure below shows an example of a recent surface characterization of a wafer with micro-heaters having an average size of about 40mm.
Laser profilometer measurement of a microheater surface after use
The design of experiments will investigate the range of parameters to characterize the micro-boiling process for various liquids, geometries and process conditions. Electrical pulse conditions, heater material and surface conditions, and bulk fluid conditions. The output variables to be observed will be the bubble formation process: the location and number of nucleation sites, growth rate, surface temperature, pressure pulse generation, and collapse phase the temperature and pressure variations in the bulk fluid inside the firing chamber, and the drop formation once ejected. Results from these studies will yield design rules and correlations for an energy efficient micro boiling process to generate the maximum possible drop momentum.
These experimental results were for water as the working fluid. Immediate goals are to extend the range of experiments to include different fluids. Not to forget that the main objective of this research is to study the boiling and drop formation processes as a consequence of very fast electrical pulses in confined spaces. To fully accomplish this main goal, specific objectives have been designed and are listed below:
Progress will be presented to the academic community through open presentations every six months. These presentations will be held at Santa Clara University, Hewlett-Packard Labs or Hewlett-Packard-Puerto Rico. Personal from Hewlett Packard, from other industries and the scientific community is invited to assist and share ideas on the progress achieved. Also, the results of the project will be widely spread among the scientific community by journal articles and conference presentations.