How to use Kelvin Forces to tune dynamically the interaction between colloids and dipolar polymers.

Using Langevin Dynamic simulations we show two examples of how the competition between entropic forces and interactions arising from the mismatch between the non-polar and dipolar moieties present in a media (Kelvin Forces) can be used to tune the interaction between colloids and polymers via an external field. Results are important for both electric and magnetic systems. In a first example [Polymers 17, 366, (2025)], we show that it is possible to create dipolar polymer brushes whose interaction with colloidal particles can be switch from a repulsive to an attractive nature by just adjusting the strength of the external field. This effect can be used to favour applications of polymer brushes in which the entrapping and retention of colloidal particles for a later release is required. In a second example [Polymers 16, 820 (2024) ], we show that the depletion interaction of colloidal particles immersed in a bath of dipolar polymers can be also modulated using an external field. In many cases it is possible to switch from typical depletion force profiles between two colloidal particles to force profiles that contain one or several stationary points depending on the strength of the external field. Furthermore, the inter-particle distance at which these stationary points occur can be also controlled to a certain extend by adjusting the strength of the external field. These findings can be useful in the development of magnetic colloidal tweezers, and colloidal ratchets. We thank the funding support of the Spanish Ministry of Economy and Competitiveness (MINECO/AEI/FEDER,UE) via grant number DPI2017-86610-P, and MCIN/AEI/10.13039/501100011033 through grants number PID2020-118317GB-I00, and , PID2021-123723OB-C22.