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Evaporation to Create Hydrophobic Coating

Controlled Evaporation Mixing (CEM) devices are useful in applying a hydrophobic coating for water-repellent fabric. Empa, a Swiss Federal Laboratory for Materials Science and Technology, employs a CEM Vapor Generation System, which integrates a liquid mass flow meter and a gas mass flow controller along with the CEM device.

The resulting system is useful for evaporating silicon organic HMDSO for material applications. In addition, Empa uses plasma polymerization to deposit thin (nanoscale) layers on top of fabrics and fibers. In this process, flow controllers are useful for adding the polymer precursors.

Hydrophobic Coating Processes

  • accurate control of gas/liquid mixture
  • stable vapor flow
  • low to high vapor flow rates

Controllable and Accurate Vapor Deposition


For hydrophobic coating processes, the CEM device evaporates the organic silicon compound HMDSO (hexamethyl-disiloxane). In this process, a container under pressure supplies liquid HMDSO at ambient temperature. Then, a liquid mass flow meter (mini CORI-FLOW series), and its mixing valve, measure the flow rate. This mixing valve in the CEM device is also responsible for introducing and mixing the liquid HMDSO with the carrier gas from a thermal mass flow controller. For example, argon mixes with the carrier gas using an EL-FLOW Select series.

Once the fluid vaporizes and mixes, it passes through the temperature-controlled heat exchanger. This enables the liquid phase to change to the vapor phase. This vapor flow is then introduced into the reactor, which operates under vacuum conditions at 7x10e-2 Torr. A supervisory system controls the entire evaporation and reaction process. Thanks to this configuration, HMDSO can vaporize over a wide range, from 1 to 30 grams per hour. As a result, it provides a very stable, precise, and consistent vapor flow with efficient regulation.

Software for Controlling Hydrophobic Coating Evaporation Processes

The evaporation process in this setup utilizes LabVIEW software for simple and effective visualization. A traditional bubbler system, which has a limited low flow rate of carrier gas and precursor, has been replaced by the currently used CEM Vapor Generation System. As a result, this transition brings several benefits.

Benefits from Using CEM Evaporation System

With the CEM Vapor Generation System, Empa achieves a higher gas yield of 50 ml/min in comparison with the previous bubbler system, which only allows 4-5 ml/min of gas flow. Additionally, the flow of the HMDSO liquid increases. Empa’s goal is to scale up the process from the laboratory scale to an industrial scale.


The CEM system currently in use at Empa is portable and compact. This mobile setup, resembling a small office table on wheels, enables easy movement between laboratories. The compact design of Bronkhorst flow instrumentation further enhances the engineering flexibility of these flow solutions.

The HMDSO setup enables the deposition of polysiloxane coatings at low temperatures. This also makes it suitable for coating textile fibers that cannot withstand high temperatures. Furthermore, Empa’s efforts to conduct plasma polymerization at low pressure aim to increase production yield by facilitating heterogeneous deposition on the fiber’s surface and reducing the amount of chemicals involved.

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