**1. Introduction**

Two-phase heat spreaders, such as heat pipes, thermal ground planes, and vapor chambers, have been widely used in various areas owing to their high heat transfer efficiency. Improving the thermal performance of the heat spreaders becomes critical for the increasing demand for cooling requirements. Here, we introduce several surface modification techniques for enhancing the heat/mass transfer on heated surfaces and its application on the heat spreaders.

As shown in **Figure 1**, a type of two-phase spreaders, vapor chamber, is composed of wick structure, casing materials, working fluid and vapor core for vapor flow spreading. The evaporator is defined as the casing material and wick structure close to the heat source, while the condenser is defined as the casing material and wick structure close to the heat sink. The heated liquid working fluid in the evaporator vaporizes, spreading to the whole space of the vapor core. The vapor of the working fluid releases heat on the condenser side and condenses into liquid. Then it returns to the evaporator by capillary pressure through wick structure,

**Figure 1.** *Schematic of a vapor chamber structure and its working mechanism.*

compensating the liquid working fluid of the evaporator. The heat pipe works similarly with the vapor chamber, but it only transfers heat in a one-dimensional way, while the vapor chamber transfers heat both in the horizontal and vertical directions.

Recently, lots of research has been conducted on the fabrication and thermal performance enhancement of the two-phase ultrathin heat spreader. Different casing materials, such as titanium [1], polymer [2, 3], and aluminum [4, 5], were used to develop the thin and light thermal ground plane. However, copper is still the most popular type of casing material for heat spreaders owing to its low cost, high thermal conductivity, and stable chemical characteristics. Thus, most investigations of different wick structures were conducted on the copper-based two-phase heat spreaders. Several types of wick structures were proposed to enhance the thermal performance of thin and ultrathin two-phase heat spreaders, including single arch-shaped sintered-grooved wick [6], bilateral arch-shaped sintered wick [7], mesh-grooved wick [6], pillar-mesh composite wick [8, 9], spiral woven mesh [10], copper fiber [11] and so on.

While many researchers focused on developing a new wick structure or modify wick structure in microscale to enhance the thermal performance of the two-phase ultrathin heat spreader, we tried to use some surface modification methods on the existing evaporator and the wick structure of the evaporator to change its wettability and structure in nanoscale for enhancing the thermal performance. The outline of the chapter is as follows. First, the mechanism of the wettability patterned surface on the nucleate boiling and the droplet evaporation are introduced, respectively. Then, an ultrathin vapor chamber with a wettability patterned evaporator is illustrated as an application case for the wettability patterned surface. After that, the fabrication of a micro/nanostructured surface is introduced and its effect on the wick structure and condenser surface is discussed. The experimental data of several types of heat spreaders with micro/nanostructured surfaces are analyzed.
