Self-Assembled Monolayer (SAM) Coatings for Condensation Enhancement

Dropwise condensation alone has shown the ability to increase condensation heat transfer coefficients by an order of magnitude over the filmwise condensation that is typical of vertically oriented condensers in thermosyphons and other two-phase systems.  By condensing on a wettability gradient surface, heat transfer coefficients are further increased over dropwise condensation by the fast removal of condensing droplets on the surface.  Unlike the filmwise and traditional dropwise condensation, the gradient surface does not require gravity or forced convection to remove liquid from the condensing surface.

A gradient surface is defined as a surface with locally varying surface energy or wettability.  The gradient surface causes a droplet to experience decreasing contact angles as the droplet travels along the path of increasing surface energy, as shown in Figure 1.  The difference in contact angle is due to locally varying properties of the condensing surface, controlled by varying surface concentrations of molecules with low surface energy.  The difference in contact angle on opposite sides of a droplet condensing on a gradient surface provides a driving force in the direction of decreasing contact angle.  The motion is sustained until the droplet reaches the end of the gradient (e.g. near an evaporator).

Figure 1.  The physics of droplet motion on a chemically induced surface gradient is illustrated.  A constant supply of liquid droplets is supplied uniformly by condensation on the gradient surface.  The droplets move towards the more hydrophilic region of the gradient surface due to the difference in contact angle experienced by opposing sides of the droplets.

Figure 1. The physics of droplet motion on a chemically induced surface gradient is illustrated. A constant supply of liquid droplets is supplied uniformly by condensation on the gradient surface. The droplets move towards the more hydrophilic region of the gradient surface due to the difference in contact angle experienced by opposing sides of the droplets.

The surface gradient is created by varying the concentration of a monomer with hydrophobic surface properties along the surface.  In one method of fabricating surface gradients, the concentration on the surface is controlled by a diffusion controlled reaction of an organosilane molecule on the surface.  An illustration of the surface modification process is shown in Figure 2.  A liquid droplet of the organosilane molecule is suspended above the silanized copper surface. The molecules evaporate from the droplet and diffuse through air, approaching the copper/silicon surface.  Once the organosilane molecules reach the surface, the reactive end of the molecule forms a chemical bond with the silicon surface.  The low surface energy end of the molecule points directly away from the surface.  Since there is a gradient in the bulk concentration (air/SAM mixture) above the surface, a gradient in the surface concentration of the molecules is developed.  This gradient in bulk concentration is due to the diffusion of the SAM molecules from the concentrated liquid SAM droplet through the dilute air phase.

Figure 2.  The fabrication of a chemically induced surface gradient is illustrated.  Organosilanes are short polymer molecules of which one end reacts with a silicon surface and the other end can possess a desirable surface characteristic, in this case a low surface energy molecule is desired.  These molecules are bonded in a diffusion controlled reaction to create a variation in concentration and therefore surface energy on a surface.   Liquid droplets formed by condensation on the surface gradient will move towards the better wetting region.

Figure 2. The fabrication of a chemically induced surface gradient is illustrated. Organosilanes are short polymer molecules of which one end reacts with a silicon surface and the other end can possess a desirable surface characteristic, in this case a low surface energy molecule is desired. These molecules are bonded in a diffusion controlled reaction to create a variation in concentration and therefore surface energy on a surface. Liquid droplets formed by condensation on the surface gradient will move towards the better wetting region.

ACT has acquired data at a saturation temperature of 100°C without non-condensable gas (i.e. not in open atmosphere).  Data were acquired at four input powers (25W, 50W, 75W and 100W) over the 2.54cm by 1.27cm heat input section (condenser heat fluxes ranged between 7.75 W/cm2 and 31 W/cm2).  Figure 3 shows data acquired from a surface oriented vertically.  The baseline filmwise mode condensation data agreed well with the Nusselt filmwise condensation model.  The near perfect agreement validates the test section and method of heat transfer coefficient measurement for this study.  Dropwise condensation on the non-wetting surface produced heat transfer coefficients that were 5-8 times higher than filmwise condensation, a significant improvement.  By condensing on the gradient surface, a 35% further improvement in the heat transfer coefficient was observed as compared to a typical dropwise condensation surface.  The expected improvement in the vertical orientation was not expected to be excessive, since gravitational force is able to pull condensing droplets on a vertical surface.   The gradient adds some additional force to move the droplets in addition to gravity, resulting in increased performance.

Figure 3.   A plot of experimental measured heat transfer coefficient results for a non-wetting, gradient, and wetting surface in the vertical orientation are shown.  The filmwise data compared well to the Nusselt model, validating the acquired data from the test section.

Figure 3. A plot of experimental measured heat transfer coefficient results for a non-wetting, gradient, and wetting surface in the vertical orientation are shown. The filmwise data compared well to the Nusselt model, validating the acquired data from the test section.

 

Another test was performed with the condenser operating in a horizontal mode with the surface oriented about 5° against gravity (condenser below evaporator); see Figure 4.  In this orientation the droplet has to be moved uphill in order to reach the end of the condensing section.  The condensation heat transfer coefficients measured were very high (~10,000W/m2K), similar to that of filmwise condensation on a vertical surface.

Figure 4. A plot of experimental measured heat transfer coefficient results for a gradient tilted 5° against gravity are shown. The non-wetting and wetting surfaces flooded as liquid was not able to return to the boiling section.

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