From the discovery of disjoining pressure to cutting-edge nanotechnology applications
Look closely at a drop of morning dew on a spiderweb, or watch how soap bubbles form perfect spheres before your eyes. These everyday miracles are governed by invisible forces operating at the nanoscale—surface forces that dictate how materials interact when they come into close contact.
Surface forces determine how lubricants reduce friction and wear in machinery, from nanoscale devices to industrial equipment.
Understanding surface interactions enables targeted drug delivery systems that improve therapeutic efficacy.
From the lubricants that keep machines running smoothly to the biological processes within our cells, surface forces influence countless aspects of our world. The science of understanding these forces has transformed from a theoretical curiosity to a foundation of modern nanotechnology, enabling breakthroughs in medicine, materials science, and energy technology.
Scientist Boris Derjaguin introduced the concept of disjoining pressure, describing the difference between the pressure of a strongly confined fluid and its corresponding bulk phase .
Building on Derjaguin's work, scientists developed the DLVO theory (named after Derjaguin, Landau, Verwey, and Overbeek) 8 .
Discovery of additional surface forces beyond DLVO theory, including hydration forces, hydrophobic forces, and steric interactions.
In the 1930s, scientist Boris Derjaguin discovered something peculiar about thin liquid films—they exhibited properties that couldn't be explained by conventional physics. He introduced the concept of disjoining pressure, which describes the difference between the pressure of a strongly confined fluid and its corresponding bulk phase .
Visualization of molecular interactions at surfaces
Building on Derjaguin's work, scientists developed what became known as the DLVO theory (named after Derjaguin, Landau, Verwey, and Overbeek) in the 1940s 8 . This theory explains how surfaces interact in liquid media through two primary forces:
A ubiquitous attractive force between all molecules that results from fluctuating electric dipoles.
A repulsive force that occurs when charged surfaces in solution attract counter-ions, forming overlapping electrical layers that push surfaces apart.
| Force Type | Origin | Range | Effect |
|---|---|---|---|
| Van der Waals attraction | Electromagnetic interactions between molecules | Short-range (decays with 1/distance⁶) | Always attractive |
| Electric double-layer repulsion | Overlapping ion clouds from charged surfaces | Long-range (decays exponentially) | Repulsive between like charges |
While the DLVO theory provides an excellent foundation, scientists have discovered additional surface forces that operate in specific situations:
Repulsive forces that occur when surfaces approach so closely that water molecules must be squeezed out from between them.
An attractive force between non-polar surfaces in water that drives them together.
When polymer-coated surfaces prevent close approach through physical obstruction.
These discoveries have paved the way for nanotechnology applications. Researchers can now design surfaces with specific properties—super-repellent coatings, targeted drug delivery systems, and novel lubricants—by manipulating these fundamental forces 3 .
How do scientists measure forces so tiny they operate on molecular scales? The breakthrough came with the development of the Surface Forces Apparatus (SFA), a remarkable instrument that can measure forces as small as 10 nanonewtons with distance resolutions better than 1 Ångstrom (0.1 nanometers) 4 5 .
The conventional SFA uses atomically smooth mica sheets glued to curved glass lenses arranged in a crossed-cylinder configuration 4 .
As surfaces approach, their separation is tracked through shifts in interference fringes, while simultaneously measuring minute forces through spring deflection 5 .
Researchers prepare atomically smooth mica surfaces, sometimes coating them with materials for study.
One surface is gradually moved toward the other while a laser monitors separation through interference patterns.
The instrument records separation distance and corresponding force, generating force-distance profiles.
Experiments conducted in various liquids, temperatures, and under electrochemical control.
| Surface Separation (nm) | Measured Force (μN/m) | Force Type | Molecular Interpretation |
|---|---|---|---|
| 50 | 0.01 | Repulsive | Beginning of double-layer overlap |
| 20 | 0.5 | Repulsive | Strong double-layer repulsion |
| 10 | 2.0 | Repulsive | Steric/hydration forces dominate |
| 5 | -1.5 | Attractive | Van der Waals attraction prevailing |
| 0.5 | -5.0 | Strong attraction | Contact, potential bond formation |
SFA measurements have revealed fascinating phenomena that challenge simple theoretical models. When liquids are confined between surfaces at nanometer separations, they can organize into layered structures, causing the force to oscillate between attractive and repulsive as surfaces approach 5 .
In one particularly important application, SFA studies have helped resolve controversies around hydration forces—additional repulsive forces that dominate at very short distances (typically decaying exponentially with a characteristic length of about 3 Å) 4 .
These forces play crucial roles in biological processes like membrane fusion 4 .
| Tool/Technique | Function | Key Features |
|---|---|---|
| Surface Forces Apparatus (SFA) | Directly measures forces between surfaces | High force (10 nN) and distance (0.1 nm) resolution; can use various surface materials |
| Atomic Force Microscope (AFM) | Measures forces between a tip and surface; can image surfaces | Can measure forces over small contact areas; allows imaging before force measurement |
| Twin-Path SFA | Measures forces between opaque samples | Enables studies of metals, polymers; allows electrochemical studies |
| Resonance Shear Measurement (RSM) | Studies nano-rheology and lubricity of confined liquids | Can monitor properties from μm separation to contact; high sensitivity |
Different techniques offer complementary approaches to studying surface forces:
Comparative capabilities of surface force measurement techniques
The development of new SFA variants continues to expand research capabilities:
The understanding of surface forces has enabled remarkable technological advances across multiple fields:
Researchers use surface force measurements to design better mucoadhesive polymers that can stick to mucosal surfaces, improving drug delivery 4 .
Understanding forces in nanoconfined liquids has led to improved lubricants that reduce friction and wear in machinery 3 .
Surface forces principles guide the creation of self-assembling materials that organize themselves into functional structures 5 .
Recent advances continue to push boundaries. The development of the twin-path SFA now allows force measurements between opaque materials like metals and polymers, opening new possibilities for studying batteries, coatings, and functional materials 5 .
Similarly, the integration of electrochemical control enables studies relevant to energy storage and conversion technologies 5 .
Emerging applications of surface forces science include:
The journey from Derjaguin's concept of disjoining pressure to modern nanotechnologies demonstrates how understanding fundamental forces can transform technology. What began as curiosity about why thin films behave strangely has evolved into a sophisticated science that enables precise control over molecular interactions.
As research continues, surface science is revealing even more subtle phenomena. Recent work has introduced the concept of "disjoining chemical potential"—a twin to disjoining pressure that arises when particles cannot exchange with their environment . This new understanding advances our ability to design materials and processes at the nanoscale.
From more efficient drug delivery systems to advanced materials with tailored properties, the applications of surface forces science continue to expand. The next time you notice a raindrop beading on a leaf or experience the smooth operation of a well-lubricated machine, remember the intricate dance of forces operating at scales far beyond what the eye can see—a testament to how understanding the smallest interactions can yield the biggest advances.