“Particle-Free” Magnetic Actuation of Droplets on Superhydrophobic Surfaces Using Dissolved Paramagnetic Salts

Abstract

Magnetic actuation is a droplet manipulation mechanism in digital microfluidics (DMF), where droplets can be actuated over a (super)hydrophobic surface with a magnetic force. Superparamagnetic particles or ferromagnetic liquids are added to the droplets to provide a "handle" by which the magnet can exert a force on the droplet. In this study, we present a novel method of magnetic manipulation, where droplets instead contain paramagnetic salts with molar magnetic susceptibilities (χ) approximately ≈10 000× < that for superparamagnetic particles. Droplet actuation is facilitated by low surface friction on fluorous silica nanoparticle-based superhydrophobic coatings, where <2 μN is required for reproducible droplet actuation. Different paramagnetic salts with χ from ≈4500 to 72 000 (× 10 cm mol) were used to make aqueous solutions of different concentration and tested for droplet actuation and sliding angle using permanent magnets (1.8-2.1 kG). Paramagnetic salts are compared in terms of solubility, minimum required concentration, and maximum droplet velocity before disengagement. There is a strong correlation between the magnetic susceptibility of the salt solution, its concentration, and ease of actuation. As an application example, droplets containing a paramagnetic salt and doxorubicin (leukemia drug) are magnetically actuated and interrogated using laser-induced fluorescence. Signal attenuation due to the MnCl salt was examined, and the Stern-Volmer quenching constant was determined.

Two-Step Numerical Approach To Predict Ferrofluid Droplet Generation and Manipulation inside Multilaminar Flow Chambers

“…negative magnetophoresis where the continuous phase is a ferrofluid) have recently been reported as well. [12][13][14] Nonetheless, the use of magnetic supports inside the droplet volume is beneficial as they may act as magnetic stirrers to improve the mixing of the droplet content which will improve the system performance for several applications. 2 Page 4 of 43…”

Magnetowetting dynamics of sessile ferrofluid droplets: a review

“…A paramagnetic droplet with dissolved salts exhibits deformation dynamics similar to an FF droplet in the magnetic field. Mats et al 153 studied the morphological evolution of a paramagnetic droplet of 10 μL volume in a magnetic environment after depositing it atop a superhydrophobic surface. It was seen that, for the paramagnetic solution of a particular salt, the base contact area of the droplet increased with an increase in the salt molar concentration, considering a similar magnetic environment.…”

“…A paramagnetic droplet with dissolved salts exhibits deformation dynamics similar to an FF droplet in the magnetic field. Mats et al 153 studied the Fig. 22 Morphology of ferrofluid marbles in the presence of a magnetic field.…”

Application of Gold Nanoparticle-Based Materials in Cancer Therapy and Diagnostics

“…Usually, PTT is carried out in two stages (Figure 5): the introduction of a photosensitizer drug into the tumor area (or vein) and its accumulation in cancer cells, after which the tumor area is irradiated with a laser with a specific wavelength [124]. The attachment of doxorubicin to the surface of AuNPs protects it from P-gp efflux, thus increasing the retention of doxorubicin in cells [105,106], and that combined with high drug loading capacity and effective drug release under pH control combined with the advantage of multimodal visualization inside cells show the high potential of this delivery system in medicine [107]. Furthermore, drug delivery systems, by binding cytotoxic drugs to the surface of AuNPs through an acid-labile bond, have demonstrated their potential to inhibit the growth of cancer cells (for instance, MCF-7/ADR) owing to the high efficiency of cellular uptake through endocytosis and subsequent acid-dependent release in cells [108].…”