Micropropulsion System

It is not necessary that the cell is located precisely at the interface. If the cell is located below the interface, the excreted surfactant will diffuse to the interface and set up gradients in surface tension that will give rise to convective flow patterns. The flow will carry the cell in the same direction, but at altered speed because of altered momentum balance. 

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In this entry, we provide an overview of the various engineering issues and challenges surrounding the modeling, design, and fabrication of micropropulsion systems and supersonic micro-nozzles in particular. From this, it is our hope and intent that the reader will develop an appreciation for the unique aspects of supersonic nozzle behavior on the microscale. 

Janson, S. et al. (1999). Micropropulsion systems for aircraft and spacecraft. In Microengineering Aerospace Systems. Ed. H. Helvajian, pp. 657-696. The Aerospace Press and AIAA, USA. 

The Marangoni force can be used as a micropropulsion system, as some bacteria actually do in nature. The principle is simple. The bacteria emit some surfactant that lowers the surface tension behind the little body. The body is pushed forward as the interface tries to minimize the region of high surface tension (without surfactant) while maximizing the region of low surface tension (with surfactant). One can build a little boat illustrating this principle by attaching a piece of soap at the end of a stick. As the soap dissolves, the stick moves forward. 

Marangoni force can be used as a micropropulsion system. In fact, some bacteria in nature move based on the Marangoni force. Surface tension phenomena occur at the interface between two immiscible fluids as a consequence of altered forces on molecules in that region. It is a force exerted on the plane of the interface and of uniform strength in all directions so that an object (bacteria) suspended in the interface is subjected to equal pull in all directions, as shown in Figure 5.33. 

Within the specific context of micro- and nanofluidics, the fabrication of self-assembled nanostrucures can represent an enabling technology for novel engineering applications. One such application - and the specific motivation for this article - lies in the creation of catalytic nanostructures for micro-Znanoscale reacting flow systems. A specific example is the catalyzed chemical decomposition of monopropellant fuels for the purposes of small satellite micropropulsion [1] (Fig. 2). Here, the size and density of nanorod formations naturally provide the high surface area-to-volume ratios desirable for efficient catalysis in microscale geometries. Certainly other important appli-... 

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