Exploring Adsorption at Nanoscale: New Insights from Optical Tweezer Electrophoresis

Optical tweezers, which earned the Nobel Prize in Physics in 2018, are a powerful tool used to manipulate microscopic particles using light.

In recent years, understanding how particles adhere to surfaces at small scales has become critical in various scientific fields, including coating, water purification, and the synthesis of food emulsions. One of the most intriguing phenomena in this area is adsorption—the process by which one material adheres to the surface of another. The adsorption process plays a vital role in industries ranging from water treatment to pharmaceuticals, yet scientists have long struggled to observe and measure this phenomenon at extremely small, dynamic time scales.

A breakthrough in this area has come from researchers at the Raman Research Institute (RRI), a leading research institution in India, who have applied an innovative technique known as optical tweezer electrophoresis to study the adsorption of clay nanoplatelets onto microspheres. This method is not only faster and more precise than traditional approaches, but it also enables real-time tracking of adsorption at unprecedented temporal and spatial scales.

Optical Tweezer Electrophoresis: A Game-Changer in Adsorption Research

Optical tweezers, which earned the Nobel Prize in Physics in 2018, are a powerful tool used to manipulate microscopic particles using light. By focusing a laser beam on a particle, scientists can exert optical forces to move, trap, and control the position of tiny objects like nanoparticles and colloidal spheres. At RRI, researchers used this technique to trap and manipulate latex spheres suspended in a clay-water mixture, allowing them to study the adsorption of Laponite clay nanoplatelets on the surfaces of the spheres.

In the experiments, the researchers applied an electric field across the suspension of clay and water and used optical tweezers to monitor the movement of the trapped latex spheres. They tracked changes in the electrical charge of the latex spheres as the clay particles adhered to them. The researchers achieved an astonishing measurement rate of 30,000 data points per second, enabling them to capture the fast kinetics of adsorption as it occurred in real time. This allowed for a much deeper understanding of the dynamic interactions between the clay nanoplatelets and the latex spheres, providing valuable insights into the adsorption process that were previously inaccessible with conventional techniques.

Tracking the Mechanisms of Adsorption in Real Time

The key to understanding adsorption lies in the ability to observe the changes in the system as they happen, and optical tweezer electrophoresis provides a unique advantage in this regard. The real-time tracking of adsorption allows researchers to monitor both the rate and extent of nanoplatelet adsorption on a single particle, offering insights into how this process is influenced by different environmental factors.

In the case of Laponite clay, the researchers observed that higher concentrations of the clay accelerated the adsorption process. This is due to the increased availability of nanoplatelets, which interact with the latex spheres more quickly as their concentration rises. This real-time observation of adsorption kinetics can greatly enhance the understanding of how adsorbates interact with surfaces, which is essential for improving various industrial processes, such as coating and water purification.

In addition to optical tweezer electrophoresis, the researchers used cryogenic-field emission scanning electron microscopy (cryo-FESEM) as a complementary technique. Cryo-FESEM allowed them to visually inspect the adsorption patterns of the clay nanoplatelets on the latex spheres. The researchers observed that when the latex spheres were suspended in pure water, there was little to no adsorption of the clay particles. However, when the latex spheres were placed in a clay-water mixture, the clay nanoplatelets adhered to the spheres in distinct patches, revealing the intricate nature of the adsorption process. The size distribution of these patches was measured to be around 50 nm for small clay aggregates and 25 nm for individual nanoplatelets, further confirming the dynamic nature of the adsorption.

Controlling the Adsorption Process: The Role of Additives

The researchers also explored how additives such as common salt and peptizing agents could influence the adsorption process. These additives were found to play a key role in controlling the interparticle interactions between the clay nanoplatelets and the latex spheres. By tuning these interactions, the researchers were able to better understand the relative contributions of different forces—electrostatic and non-electrostatic dispersion forces—that govern the adsorption process.

At higher ionic concentrations, the electrostatic interactions became more pronounced, accelerating the adsorption of nanoplatelets, aggregates, and gel network strands. The researchers concluded that non-electrostatic dispersion interactions were the dominant mechanism driving the initial adsorption of the clay particles. As the ionic strength of the system increased, electrostatic screening led to faster adsorption, revealing the complex interplay between dispersion and electrostatic forces.

The Potential for Advanced Applications

The findings from this research have important implications for several industries that rely on adsorption processes. For example, in the field of water purification, understanding the dynamics of particle adsorption can help improve the efficiency of removing pollutants from water. Similarly, in the food industry, controlling the adsorption of colloidal particles can enhance the stability of emulsions, which are critical in many food products.

The researchers' work also holds promise for advancing the development of microfluidic devices and other technologies that require precise control of particle interactions. As Prof. Ranjini Bandyopadhyay, lead of the RheoDLS lab at RRI, explained, their next goal is to integrate optical tweezer electrophoresis with microfluidic techniques to trap multiple particles simultaneously. This will enable them to investigate medium-mediated charge transfer between different particles and further enhance the precision of their measurements.

Looking Ahead: The Future of Adsorption Research

By combining optical tweezer electrophoresis with advanced imaging techniques such as cryo-FESEM, researchers are opening up new avenues for understanding adsorption at the nanoscale. This work provides not only valuable insights into the mechanisms of clay nanoplatelet adsorption but also sets the stage for the development of more efficient industrial processes and products.

As the researchers continue to refine their techniques and expand the scope of their investigations, the potential applications of their findings will only grow. From improving water purification systems to enhancing the stability of food emulsions, the ability to control and optimize adsorption at the molecular level could lead to significant advances in a variety of industries, offering better products and more sustainable solutions for the future.

With this research, the Raman Research Institute has laid the foundation for a new era of adsorption science, where precision and real-time observation are key to unlocking the full potential of nanoparticle interactions in a wide range of applications.