When hydraulic oil is mixed with water, does its viscosity become thinner or thicker?
The common understanding is that “oil mixed with water = thinner!”
This is only half true. Once emulsification occurs, the viscosity will defy common sense and skyrocket.
1. First, debunk the common misconception that “oil mixed with water will always become thinner”: This only applies to situations where there is “no emulsification.”
Why do people think that “oil mixed with water becomes thinner”? Because in everyday life, “oil + water” is usually in a “layered state”—for example, if water is added to rapeseed oil, the water sinks to the bottom, and the oil remains the same consistency, or even slightly thinner overall due to the addition of “low-viscosity water.”
However, in hydraulic systems, “oil + water” can have two completely different outcomes, only one conforming to “common sense,” while the other is exactly the opposite:
①: No emulsification (consistent with common sense) – Slight thinning, minimal impact.
If very little water is mixed in, and the hydraulic oil doesn’t contain enough surfactants (such as antifoaming agents or rust inhibitors), the water will exist as large droplets (over 100μm in diameter, visible to the naked eye). Because its density is greater than oil, it will quickly sink to the bottom of the tank (what we commonly call “free water”).
This aligns with common sense: water acts as a “low-viscosity impurity,” at most reducing the hydraulic oil’s viscosity from 46cSt (a unit of viscosity, the higher the number, the more viscous) to around 40cSt. The change is very slight, and the oil will essentially return to normal after draining the water.
②: Emulsification (counterintuitive) – Directly thickens, viscosity increases several times.
However, once the water content is slightly higher, and it encounters antifoaming agents and rust inhibitors in the oil (these are all “surfactants”), the situation is completely reversed:
The surfactants act like “glue,” breaking the oil into tiny droplets of 1-10μm (10 times thinner than a human hair), allowing them to be evenly “soaked” in water, forming a milky white “oil-in-water emulsion.”
At this point, the oil is no longer “a little thin,” but “absurdly viscous”—46cSt oil can spike to 200cSt, the feel changes from “clear oil” to “thick porridge,” and pumping it feels like “suctioning paste,” with flow resistance reaching its maximum.
2. Key: Why does “emulsification” break the “common sense of thinning”? 3 counterintuitive core reasons.
“When oil is broken down into small droplets and mixed with water, why does it thicken instead?” — This is the key to the counterintuitive question. The answer lies in the “special structure” of the emulsion, which is completely different from “simple layering”.
(1) Surfactants did not “help” but instead added a “friction pad”. Everyone thought that surfactants only “mixed oil and water together”, but they did not expect that they would “block” the flow: antifoaming agents (such as polyethers) and rust inhibitors (such as carboxylic acid soaps) in hydraulic oil have “lipophilic” molecules at one end and “hydrophilic” molecules at the other end. When an emulsion is formed, they will coat the oil droplets and water with a dense “interfacial film” (10-100nm thick, thinner than plastic wrap).
(2) Small oil droplets are not “impurities” but “micro-obstacles”. Common sense tells us that “water is a low-viscosity impurity”, but emulsified oil droplets become “small pebbles blocking the road”: unemulsified water is a “large droplet” that sinks to the bottom and does not block the road; but oil droplets of 1-10μm are about the same size as the gap of a hydraulic pump (usually 5-20μm) and are evenly dispersed in the water – just like countless millet grains scattered in a “flowing waterway”.
3. Final Reminder: Don’t be held back by “common sense”! The dangers of emulsification are greater than you think.
Because of the common misconception that “oil mixed with water will become thinner,” many people fail to address emulsification issues promptly, ultimately leading to major malfunctions: viscous emulsions make it difficult for pumps to draw oil, easily causing cavitation; they also dilute anti-wear additives—the additives in the oil droplets are “locked” in small droplets, unable to act on the friction surfaces of the plunger and cylinder, ultimately leading to direct metal wear and shortening component life by more than half. Next time you find water in your hydraulic oil, don’t think, “It’ll definitely become thinner, just drain the water”—first check if the oil has turned milky white and feels sticky: if so, it’s emulsified and needs immediate attention!