Xiaopeng Qu

Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate

The self-cleaning function of superhydrophobic surfaces is conventionally attributed to the removal of contaminating particles by impacting or rolling water droplets, which implies the action of external forces such as gravity. Here, we demonstrate a unique self-cleaning mechanism whereby the contaminated superhydrophobic surface is exposed to condensing water vapor, and the contaminants are autonomously removed by the self-propelled jumping motion of the resulting liquid condensate, which partially covers or fully encloses the contaminating particles. The jumping motion off the superhydrophobic surface is powered by the surface energy released upon coalescence of the condensed water phase around the contaminants. The jumping-condensate mechanism is shown to spontaneously clean superhydrophobic cicada wings, where the contaminating particles cannot be removed by gravity, wing vibration, or wind flow. Our findings offer insights for the development of self-cleaning materials.

Self-propelled sweeping removal of dropwise condensate

Dropwise condensation can be enhanced by superhydrophobic surfaces on which the condensate drops spontaneously jump upon coalescence. However, the self-propelled jumping in prior reports is mostly perpendicular to the substrate. Here, we propose a substrate design with regularly spaced micropillars. Coalescence on the sidewalls of the micropillars leads to self-propelled jumping in a direction nearly orthogonal to the pillars and therefore parallel to the substrate. This in-plane motion in turn produces sweeping removal of multiple neighboring drops. The spontaneous sweeping mechanism may greatly enhance dropwise condensation in a self-sustained manner.

Hotspot cooling with jumping-drop vapor chambers

Hotspot cooling is critical to the performance and reliability of electronic devices, but existing techniques are not very effective in managing mobile hotspots. We report a hotspot cooling technique based on a jumping-drop vapor chamber consisting of parallel plates of a superhydrophilic evaporator and a superhydrophobic condenser, where the working fluid is returned via the spontaneous out-of-plane jumping of condensate drops. While retaining the passive nature of traditional vapor-chamber heat spreaders (flat-plate heat pipes), the jumping-drop technique offers a mechanism to address mobile hotspots with a pathway toward effective thermal transport in the out-of-plane direction.

Thermal Bubble Dynamics Under the Effects of an Acoustic Field

In this research, dynamics of a micro thermal bubble existing in an acoustic field have been studied by a high-speed camera and a micro temperature sensor. The micro thermal bubble was generated by a micro heater, which was fabricated by the MEMS (micro-electro-mechanical system) technique and packed into a transparent mini chamber. The acoustic field inside the chamber was generated by a piezoelectric plate that was attached on the top side of the chambers wall. Compared with micro thermal bubble dynamics in normal conditions, several different bubble dynamic phenomena in acoustic conditions have been found, such as bubble departure and attraction around the heater, bubble oscillating in the liquid volume, etc. By theoretical analysis, the main mechanism of bubble movements is attributed to the balance between Marangoni force and acoustic force. All these bubble dynamic phenomena improve the liquid convective flow and enhance the heat and mass transfer. Thus, this investigation about acoustic thermal bubble dynamics may find some potential applications in micro fluid devices for different functions, such as heat/mass transfer enhancement, micro electronic cooling, micro heater protection, etc. Temperature measurement in both normal conditions and acoustic conditions confirmed that the heat transfer was enhanced by the acoustic field.

Acoustically driven micro-thermal-bubble dynamics in a microspace

In this paper, vapor-bubble dynamics confined in a microspace under the effect of an acoustic field, defined as acoustic-thermal-bubble dynamics, is investigated not only for theoretical understanding of this complex bubble dynamic phenomenon but also for development of new microfluidic devices. The micro thermal bubble is generated by a microheater which is fabricated by a standard MEMS (microelectromechanical system) technique and integrated into a transparent channel. Using a high-speed digital camera, the thermal-bubble dynamics is studied qualitatively under two different conditions: normal condition and acoustic condition. The fluid is stationary for the experiments and the effect of the flow rate is not determined in the current research. Through theoretical analysis, the whole complex bubble dynamic process under two conditions can be roughly divided into four steps: (1) bubble generation, (2) satellite bubble movement, (3) bubble evolution and (4) bubble shrinkage/removal. The effects of acoustic vibration on all these four steps are found to be distinctly different. The mechanisms behind these effects are examined by analyzing the high-speed visualization results of two-phase flow phenomena. The current experimental investigation has a number of potential applications in the development of novel microfluidic devices.

Bubble dynamics under a horizontal micro heater array

A micro heater array has been fabricated using a standard micromachining technique to study the micro thermal bubble dynamics on a horizontal downward-facing surface. Several different types of bubble dynamic phenomena, such as bubble sweeping, bubble departure/retraction and multibubbles interaction/coalescence, have been investigated utilizing a high-speed photography system. The effects of Marangoni force, buoyancy force and drag force on the bubble dynamic phenomena have been studied utilizing experimental data. The results obtained from this study are not only helpful for understanding the microscale thermal bubble dynamics but also useful for optimization of micro cooling equipment and development of novel thermal bubble-based MEMS (micro-electro-mechanical-system) devices.

Effects of acoustic vibration on microheater-induced vapor bubble incipience in a microchannel

A simultaneous temperature measurement and high-speed visualization study has been carried out to investigate the effects of acoustic vibration on vapor bubble dynamics and flow boiling in a single microchannel which is formed by bonding a bottom Pyrex glass wall with a top PDMS (polydimethylsiloxane) wall. A single platinum serpentine microheater, fabricated on the Pyrex glass substrate and integrated into the microchannel, is used to generate vapor bubbles and measure local wall temperature simultaneously. The temperature variations and flow pattern maps in terms of acoustic frequency and acoustic signal amplitude are presented, respectively. It is found that the acoustic vibration can induce bubble incipience with a relatively low heating power. The frequency, acoustic signal amplitude and flow rate are important factors affecting vapor bubble incipience. A lower acoustic signal amplitude can promote bubble incipience to happen easily but no obvious effect when further increasing the acoustic amplitude to a relatively high level. A low flow rate can advance bubble incipience, while a very high flow rate can hinder bubble incipience under the same acoustic and heating conditions. The experimental results are instructive for the design of novel microfluid and MEMS (microelectromechanical systems) devices, such as lab-on-a-chip or thermal management devices.

Acoustic Driven Micro Thermal Bubble Dynamics in a Microchannel

Understanding the effects of acoustic vibration on micro thermal bubble dynamics in a microchannel is the key to develop acoustic-thermal-bubble based microfluidic devices. For that purpose, in the current research, a series of experiments were carried out to study the acoustic-thermal-bubble dynamics in a microchannel. The thermal bubble was generated by a micro heater which was fabricated by MEMS (Micro-Electro-Mechanical-System) technique. Using a high-speed digital camera, the thermal bubble dynamics was studied in two different conditions: normal condition and acoustic condition. Through theoretical analysis, the whole bubble dynamic process in two conditions can be roughly divided into four steps, which are bubble nucleation, satellite bubbles movement, bubble evolution, and bubble shrinkage and remove. The effects of acoustic vibration on all these four steps were found to be significant. The mechanisms behind these effects are discussed by analyzing the high speed video recording results. The current experimental investigation has some potential applications in microfluidic devices, and a prototype of micro mixer based on acoustic-thermal-bubble was successfully tested.Copyright

Thermal Bubble Dynamics under the Effect of Acoustic Vibration

The effect of acoustic field on the dynamics of micro thermal bubble is investigated in this paper. The micro thermal bubbles were generated by a micro heater which was fabricated by standard Micro-Electro-Mechanical-System (MEMS) technology and integrated into a mini chamber. The acoustic field formed in the mini chamber was generated by a piezoelectric plate which was adhered on the top side of the chamber’s wall. The dynamics and related heat transfer induced by the micro heater generated vapor bubble with and without the existing of acoustic field were characterized by a high speed photograph system and a micro temperature sensor. Through the experiments, it was found that in two different conditions, the temperature changing induced by the micro heater generated vapor bubble was significantly different. From the analysis of the high speed photograph results, the acoustic force induced micro thermal bubble movements, such as forcibly removing, collapsing and sweeping, were the main effects of acoustic enhanced boiling heat transfer. The experimental results and theoretical analysis were helpful for understanding of the mechanisms of acoustic enhanced boiling heat transfer and development of novel micro cooling devices.Copyright