Knudsen pumps: a review

Abstract

The Knudsen pump (KP) is a kind of micro-pump that can form thermally induced flows induced by temperature fields in rarefied gas environments. It has the advantages of having no moving parts, simple structure, easy construction and extension, a wide range of energy sources, and low energy consumption. With the development of Micro/Nano Electro Mechanical Systems (MEMS/NEMS), extensive studies have been conducted on KPs, and the applications of KPs have widened. In order to obtain efficient flow fields in KPs, it is necessary to adopt modern computational methods for simulation and analysis. In many circumstances, the simulation and experimental results have good agreement. However, there seems to be no comprehensive review on KPs at present. In this paper, KPs are first defined and classified according to the flow mechanisms of the thermally induced flows. Then, the three aspects of configurations, performance, and applications of KPs in the current state of research are reviewed and analyzed. Finally, the current problems of KP are discussed, and some suggestions are provided for future research and applications.

Stretchable and High‐Performance Fibrous Sensors Based on Ionic Capacitive Sensing for Wearable Healthcare Monitoring

“…Using a similar mechanism, we sewed the sensors into wearable fabrics at the wrist, elbow, and knee, achieving motion detections of different human joints, as illustrated in Figure 5f–h, demonstrating the advantages of fiber‐based sensors for easy integration into wearable fabrics. In conclusion, the SICF‐based strain sensor exhibits enhanced sensitivity and a broader detection range compared to similar works, [ 14,23,24,26,46 ] attributed to the high capacitive contribution of the EDL and the excellent stretchability of the fibers, as illustrated in Figure 5i and Table S1 (Supporting Information).…”

Stretchable and High‐Performance Fibrous Sensors Based on Ionic Capacitive Sensing for Wearable Healthcare Monitoring

“…Using a similar mechanism, we sewed the sensors into wearable fabrics at the wrist, elbow, and knee, achieving motion detections of different human joints, as illustrated in Figure 5f–h, demonstrating the advantages of fiber‐based sensors for easy integration into wearable fabrics. In conclusion, the SICF‐based strain sensor exhibits enhanced sensitivity and a broader detection range compared to similar works, [ 14,23,24,26,46 ] attributed to the high capacitive contribution of the EDL and the excellent stretchability of the fibers, as illustrated in Figure 5i and Table S1 (Supporting Information).…”

Biomimetic Light‐Driven Aerogel Passive Pump for Volatile Organic Pollutant Removal

“…In brief, although the NFC membrane displays the highest flow rate owing to large pores, the PMSQ membrane allows for stronger irradiances due to its superior thermal stability (Fig- ure 3D). In terms of power consumption, our aerogel gas pumps deliver sustainable pumping flow rates around 3.2-10.0 μL min −1 mm −2 at a solar power input of 0.69-1.57 W, and they exhibit improved performance compared to reported Knudsen pumps [9] as well as traditional mechanical displacement, electro-or magnetokinetic micropumps (Figure 2F). The superior pumping performances can lead to an efficient mass transfer of gas pollutants (e.g., VOCs) into the porous aerogels and subsequently toward the catalytically active sites of the absorber layer as demonstrated in the following section.…”

“…However, if the pores are too large, the Knudsen effect is reduced and the flow rate also decreases due to the improper ratio between the gas mean free path and the pore size. [ 9 ] Therefore, an intermediate range of pore sizes, i.e., Knudsen numbers, exists for the maximum flow rate. When we compare the NFC, UMCh, and PMSQ aerogels with 10 wt% MnO 2 , the flowrate of the PMSQ aerogel is the lowest (Figure 2E).…”

Electronic CPA‐Laser Having Enhanced Sensitivity and Tunability

“…Such a result is hard to observe in many flexible and wireless electronic sensing systems. [ 52 ] After 15 min of aerobic exercise, the volunteer's HR rises to ∼174 bpm (Figure 5j), which can be a direct measure of the cardiopulmonary function. In parallel, we have also performed the monitoring of respiration by attaching the epidermal sensor to the throat (Figure 5k) to detect the pressure variations induced by neck musculature dynamics during the respiratory cycle.…”

Stretchable and High‐Performance Fibrous Sensors Based on Ionic Capacitive Sensing for Wearable Healthcare Monitoring

“…Using a similar mechanism, we sewed the sensors into wearable fabrics at the wrist, elbow, and knee, achieving motion detections of different human joints, as illustrated in Figure 5f–h, demonstrating the advantages of fiber‐based sensors for easy integration into wearable fabrics. In conclusion, the SICF‐based strain sensor exhibits enhanced sensitivity and a broader detection range compared to similar works, [ 14,23,24,26,46 ] attributed to the high capacitive contribution of the EDL and the excellent stretchability of the fibers, as illustrated in Figure 5i and Table S1 (Supporting Information).…”

Biomimetic Light‐Driven Aerogel Passive Pump for Volatile Organic Pollutant Removal

“…In brief, although the NFC membrane displays the highest flow rate owing to large pores, the PMSQ membrane allows for stronger irradiances due to its superior thermal stability (Fig- ure 3D). In terms of power consumption, our aerogel gas pumps deliver sustainable pumping flow rates around 3.2-10.0 μL min −1 mm −2 at a solar power input of 0.69-1.57 W, and they exhibit improved performance compared to reported Knudsen pumps [9] as well as traditional mechanical displacement, electro-or magnetokinetic micropumps (Figure 2F). The superior pumping performances can lead to an efficient mass transfer of gas pollutants (e.g., VOCs) into the porous aerogels and subsequently toward the catalytically active sites of the absorber layer as demonstrated in the following section.…”

“…However, if the pores are too large, the Knudsen effect is reduced and the flow rate also decreases due to the improper ratio between the gas mean free path and the pore size. [ 9 ] Therefore, an intermediate range of pore sizes, i.e., Knudsen numbers, exists for the maximum flow rate. When we compare the NFC, UMCh, and PMSQ aerogels with 10 wt% MnO 2 , the flowrate of the PMSQ aerogel is the lowest (Figure 2E).…”

Electronic CPA‐Laser Having Enhanced Sensitivity and Tunability

“…Such a result is hard to observe in many flexible and wireless electronic sensing systems. [ 52 ] After 15 min of aerobic exercise, the volunteer's HR rises to ∼174 bpm (Figure 5j), which can be a direct measure of the cardiopulmonary function. In parallel, we have also performed the monitoring of respiration by attaching the epidermal sensor to the throat (Figure 5k) to detect the pressure variations induced by neck musculature dynamics during the respiratory cycle.…”

Stretchable and High‐Performance Fibrous Sensors Based on Ionic Capacitive Sensing for Wearable Healthcare Monitoring

“…Using a similar mechanism, we sewed the sensors into wearable fabrics at the wrist, elbow, and knee, achieving motion detections of different human joints, as illustrated in Figure 5f–h, demonstrating the advantages of fiber‐based sensors for easy integration into wearable fabrics. In conclusion, the SICF‐based strain sensor exhibits enhanced sensitivity and a broader detection range compared to similar works, [ 14,23,24,26,46 ] attributed to the high capacitive contribution of the EDL and the excellent stretchability of the fibers, as illustrated in Figure 5i and Table S1 (Supporting Information).…”

Biomimetic Light‐Driven Aerogel Passive Pump for Volatile Organic Pollutant Removal

“…In brief, although the NFC membrane displays the highest flow rate owing to large pores, the PMSQ membrane allows for stronger irradiances due to its superior thermal stability (Fig- ure 3D). In terms of power consumption, our aerogel gas pumps deliver sustainable pumping flow rates around 3.2-10.0 μL min −1 mm −2 at a solar power input of 0.69-1.57 W, and they exhibit improved performance compared to reported Knudsen pumps [9] as well as traditional mechanical displacement, electro-or magnetokinetic micropumps (Figure 2F). The superior pumping performances can lead to an efficient mass transfer of gas pollutants (e.g., VOCs) into the porous aerogels and subsequently toward the catalytically active sites of the absorber layer as demonstrated in the following section.…”

“…However, if the pores are too large, the Knudsen effect is reduced and the flow rate also decreases due to the improper ratio between the gas mean free path and the pore size. [ 9 ] Therefore, an intermediate range of pore sizes, i.e., Knudsen numbers, exists for the maximum flow rate. When we compare the NFC, UMCh, and PMSQ aerogels with 10 wt% MnO 2 , the flowrate of the PMSQ aerogel is the lowest (Figure 2E).…”

Electronic CPA‐Laser Having Enhanced Sensitivity and Tunability

“…Such a result is hard to observe in many flexible and wireless electronic sensing systems. [ 52 ] After 15 min of aerobic exercise, the volunteer's HR rises to ∼174 bpm (Figure 5j), which can be a direct measure of the cardiopulmonary function. In parallel, we have also performed the monitoring of respiration by attaching the epidermal sensor to the throat (Figure 5k) to detect the pressure variations induced by neck musculature dynamics during the respiratory cycle.…”