Droplet slipperiness despite surface heterogeneity at molecular scale

Area of Science:

• Surface science
• Tribology
• Physical chemistry

Background:

• Surface heterogeneity is traditionally linked to increased droplet friction and contact angle hysteresis.
• Understanding droplet-surface interactions is crucial for various applications, including microfluidics and coatings.

Purpose of the Study:

• To challenge the premise that surface heterogeneity increases droplet friction at the molecular level.
• To investigate the relationship between chemical heterogeneity, droplet friction, and contact angle hysteresis.

Main Methods:

• Tuning the coverage of self-assembled monolayers (SAMs) on a substrate.
• Measuring water contact angles and contact angle hysteresis.
• Utilizing molecular dynamics simulations to probe interfacial water behavior.

Main Results:

• Low contact angle hysteresis and friction were observed for both low-coverage hydrophilic and high-coverage hydrophobic SAMs, despite significant chemical heterogeneity.
• This low friction was unexpected given the presence of uncoated substrate areas larger than water molecules.
• Molecular dynamics simulations identified the mobility of interfacial water molecules as the key factor for low friction.

Conclusions:

• The study reveals a counterintuitive mechanism where chemical heterogeneity can lead to droplet slipperiness.
• Interfacial water molecule mobility is identified as a primary driver of low contact line friction.
• Findings open new possibilities for designing surfaces with enhanced droplet mobility.