Hydrophilic or Hydrophobic?

Understanding the Hydrophilic and Hydrophobic Logic on the Material Surface

Yancheng Ruize Pigment Co., Ltd.

When conducting experiments such as film, electrode, and coating tests, many people use a contact angle photo to determine whether the material surface is "hydrophilic" or "hydrophobic".

However, whether the water droplet spreads out or rolls off, and whether it penetrates, are actually different issues. Just looking at the static shape is prone to missing adhesion, lag, roughness, reaction environment and time effects.

Ruize Pigment has always regarded the surface wetting behavior as a key part of formula adjustment during the development of hydrophilic pigments, water-repellent pigments, films, and functionalization of non-woven fabrics. Today, we have compiled six core understandings to clarify common misconceptions related to contact angles and provide references for material selection, formula iteration and performance verification.

1. Clearly explain the wetting degree

01 Wetting degree ≠ Liquid droplet adhesion

Generally, a water contact angle less than 90° is more hydrophilic, and greater than 90° is more hydrophobic. However, "forming droplets" does not equal "easily rolling off". Water droplets on lotus leaves can roll off, while some water droplets on petals may stick.

High contact angle does not mean low adhesion. When evaluating surface performance, one cannot only look at the shape of the liquid droplet, but also consider its mobility, slippability, and stickiness.

02 Surface chemistry determines how water approaches

The chemical properties of the material surface directly affect the interaction between water and the interface.

Hydroxyl, carboxyl and other polar groups are more likely to form hydrogen bonds with water molecules, which usually favors wetting; PTFE, unmodified PDMS, etc. exhibit strong hydrophobicity.

However, note: Hydrophilicity does not equal positive charge. Surface charge is affected by pH, ionic environment, cleaning state and modification process. When judging, do not confuse "wetting" with "surface charge".

03 Coarser, not necessarily more waterproof

The influence of surface roughness on wetting depends on whether the liquid enters the texture.

Wenzel state: The liquid penetrates the surface texture, and the roughness amplifies the original wetting tendency. The material was originally hydrophilic, and after roughening, it may be more prone to wetting. Cassie–Baxter state: The texture traps air, and the liquid simultaneously contacts the solid and air, which may enhance the water-repellency.

However, the air layer is not stable. Pressure, impact and prolonged contact can all change the interface state. Therefore, roughness is not simply "the rougher, the more waterproof".

04 Electrode wetting, consider the reaction scenario

The electrode surface cannot be evaluated solely by "hydrophilic" or "hydrophobic".

Hydrogen evolution, oxygen evolution and other gas evolution reactions require the electrolyte to approach the active site and for gas bubbles to be promptly expelled; carbon dioxide reduction and other gas supply reactions require the retention of gas transmission channels to avoid liquid submerging the interface.

Therefore, "dual wetting" is not a universal answer. The hydrophilicity in the air is not directly representative of whether water bubbles are easily detached underwater. Electrode wetting must be judged based on the specific reaction environment.

05 Separation and transportation, have preconditions

In separation membranes and droplet transportation scenarios, wetting performance is often applied, but the effect is not automatically established.

Water pre-wetted hydrophilic or water-underlying hydrophobic systems can achieve water permeability and oil repellency under certain conditions, but pore size, pressure difference, pollution and surface conditions will affect the result. Wetting gradient or external driving can guide the movement of droplets; however, an evenly hydrophilic track does not necessarily ensure continuous water transport.

Separation and transportation have preconditions. Judging only by the "hydrophilic" or "hydrophobic" nature of the material surface is prone to ignoring structure, operating conditions and long-term stability.

06 What else to measure besides the contact angle?

Static contact angle is only the shape of the liquid droplet at a certain moment. To more completely evaluate wetting, it is necessary to pay attention to:

Sliding angle: When does the droplet start to move;

Contact angle lag: Judging the strength of adhesion through the forward and backward angles;

Time and medium: Observing whether the droplet penetrates, evaporates or deforms;

Test conditions: Droplet volume, liquid composition, temperature and humidity, surface treatment and reading time.

The disappearance of water droplets on porous materials may be due to infiltration or evaporation. It cannot be directly determined as "becoming hydrophilic". Looking at the shape, as well as the movement, time and environment, a more complete understanding is needed.

II. Wetting property is not a label

The contact angle is intuitive, but it can also be easily misinterpreted.

A single droplet photo can tell us the instantaneous shape of the droplet, but it cannot alone indicate whether the material is easy to prevent fogging, whether it is conducive to drainage, whether it is suitable for separation, or whether it is conducive to bubble detachment.

A truly reliable judgment requires viewing the static contact angle, sliding angle, contact angle lag, surface chemistry, roughness, working conditions and time effect together.

For the development of films, non-woven fabrics, coatings, electrodes and porous materials, wetting property is more like a set of interface language rather than a simple "hydrophilic / hydrophobic" label.

For many years, Ruize pigment masterbatches have been continuously refining hydrophilic and hydrophobic functional masterbatch solutions for non-woven fabrics, films, and fiber-based substrates. In the process of formula development and customer technical services, they have continuously focused on the performance changes brought by interface wetting, relying on wetting ultrafine grinding and nano ultra-dispersal technology, providing stable and controllable functional modification solutions for the medical and health care, filtration and separation, and packaging films sectors.

If you have hydrophilic or hydrophobic functional masterbatches, or need formula debugging and sample testing related to film and non-woven fabric modification, please feel free to communicate and exchange with us.