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Measuring the Interaction Force Between a Droplet and a Super-hydrophobic Substrate by the Optical Lever Method
Published on: June 14, 2019
The search for the hydrophobic force law
Malte U Hammer1, Travers H Anderson, Aviel Chaimovich
1Department of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA.
Faraday Discussions
|November 4, 2010
Summary
Researchers propose a three-regime hydrophobic force law, explaining interactions between hydrophobic surfaces across different distances. This model addresses complex experimental data and offers a new framework for understanding hydrophobic interactions (HI).
Area of Science:
- Physical Chemistry
- Surface Science
- Colloid Science
Background:
- Hydrophobic interaction (HI) research spans nearly 30 years, yet a definitive force law remains elusive.
- Experimental data for hydrophobic forces are often inconsistent across different surface scales (nano-, micro-, macroscopic).
- Existing theories like continuum van der Waals (vdW) forces do not fully explain observed strong attractive forces at intermediate ranges.
Purpose of the Study:
- To propose a comprehensive three-regime hydrophobic force law accounting for experimental observations.
- To elucidate the complex nature of hydrophobic forces, dependent on multiple parameters.
- To provide a theoretical framework for understanding hydrophobic association across various length scales.
Main Methods:
- Analysis of existing experimental data from surface force apparatus (SFA) and other techniques.
- Development of a simplified, spherically-symmetric water model for theoretical studies.
- Comparison of experimental force-distance relationships with continuum vdW theory.
Main Results:
- A three-regime force law is proposed: electrostatic/bridging forces (100-200 Å to thousands Å), a long-range hydrophobic force (approx. 150 to 15 Å) possibly linked to water's polarizability, and a short-range hydrophobic force (<15 Å) due to water structuring.
- Observed forces, especially adhesive ones, are stronger and exhibit different distance dependence than predicted by Lifshitz theory for non-conducting dielectrics.
- The simplified water model captures key features of hydrophobic association relevant to SFA experiments.
Conclusions:
- Hydrophobic forces are complex and likely arise from a combination of fundamental interactions across different regimes.
- The proposed three-regime model offers a more complete explanation for experimentally observed hydrophobic force-distance relationships.
- Further theoretical and experimental work is needed to fully understand the 'pure' hydrophobic forces and validate the proposed model.
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