Editing Microbotics (section)

=== At Air-Fluid Interface locomotion ===
In the specific instance when microrobots are at the air-fluid interface, they can take advantage of surface tension and forces provided by capillary motion. At the point where air and a liquid, most often water, come together, it is possible to establish an interface capable of supporting the weight of the microrobots through the work of surface tension. Cohesion between molecules of a liquid creates surface tension, which otherwise creates ‘skin’ over the water’s surface, letting the microrobots float instead of sinking. Through such concepts, microrobots could perform specific locomotion functions, including climbing, walking, levitating, floating, and or even jumping, by exploring the characteristics of the air-fluid interface.<ref name=":1" /><ref>{{Cite journal |last1=Koh |first1=Je-Sung |last2=Yang |first2=Eunjin |last3=Jung |first3=Gwang-Pil |last4=Jung |first4=Sun-Pill |last5=Son |first5=Jae Hak |last6=Lee |first6=Sang-Im |last7=Jablonski |first7=Piotr G. |last8=Wood |first8=Robert J. |last9=Kim |first9=Ho-Young |last10=Cho |first10=Kyu-Jin |date=2015-07-31 |title=Jumping on water: Surface tension–dominated jumping of water striders and robotic insects |url=https://www.science.org/doi/10.1126/science.aab1637 |journal=Science |language=en |volume=349 |issue=6247 |pages=517–521 |doi=10.1126/science.aab1637 |bibcode=2015Sci...349..517K |issn=0036-8075|url-access=subscription }}</ref>

Due to the surface tension ,σ, the buoyancy force, F<sub>b</sub>, and the curvature force, F<sub>c</sub>, play the most important roles, particularly in deciding whether the microrobot will float or sink on the surface of the liquid. This can be expressed as

<math>\sigma=F_b+F_c</math>

F<sub>b</sub> is obtained by integrating the hydrostatic pressure over the area of the body in contact with the water. In contrast, F<sub>c</sub> is obtained by integrating the curvature pressure over this area or, alternatively, the vertical component of the surface tension, <math>\sigma\sin\theta</math>, along the contact perimeter.<ref>{{Cite journal |last1=Hu |first1=David L. |last2=Chan |first2=Brian |last3=Bush |first3=John W. M. |date=August 2003 |title=The hydrodynamics of water strider locomotion |url=https://www.nature.com/articles/nature01793 |journal=Nature |language=en |volume=424 |issue=6949 |pages=663–666 |doi=10.1038/nature01793 |pmid=12904790 |bibcode=2003Natur.424..663H |issn=0028-0836|url-access=subscription }}</ref>

One example of a climbing, walking microrobot that utilizes air-fluid locomotion is the Harvard Ambulatory MicroRobot with Electroadhesion (HAMR-E).<ref name=":2">{{Cite journal |last1=de Rivaz |first1=Sébastien D. |last2=Goldberg |first2=Benjamin |last3=Doshi |first3=Neel |last4=Jayaram |first4=Kaushik |last5=Zhou |first5=Jack |last6=Wood |first6=Robert J. |date=2018-12-19 |title=Inverted and vertical climbing of a quadrupedal microrobot using electroadhesion |url=https://www.science.org/doi/10.1126/scirobotics.aau3038 |journal=Science Robotics |language=en |volume=3 |issue=25 |doi=10.1126/scirobotics.aau3038 |pmid=33141691 |issn=2470-9476|url-access=subscription }}</ref> The control system of HAMR-E is developed to allow the robot to function in a flexible and maneuverable manner in a challenging environment. Its features include its ability to move on horizontal, vertical, and inverted planes, which is facilitated by the electro-adhesion system. This uses electric fields to create electrostatic attraction, causing the robot to stick and move on different surfaces.<ref>{{Cite journal |last1=Rajagopalan |first1=Pandey |last2=Muthu |first2=Manikandan |last3=Liu |first3=Yulu |last4=Luo |first4=Jikui |last5=Wang |first5=Xiaozhi |last6=Wan |first6=Chaoying |date=July 2022 |title=Advancement of Electroadhesion Technology for Intelligent and Self-Reliant Robotic Applications |url=https://onlinelibrary.wiley.com/doi/10.1002/aisy.202200064 |journal=Advanced Intelligent Systems |language=en |volume=4 |issue=7 |doi=10.1002/aisy.202200064 |issn=2640-4567}}</ref> With four compliant and electro-adhesion footpads, HAMR-E can safely grasp and slide over various substrate types, including glass, wood, and metal.<ref name=":2" /> The robot has a slim body and is fully posable, making it easy to perform complex movements and balance on any surface.