Active matter can form various collective motions. In dry and repulsive systems, a uniform polar fluid emerges in the presence of an aligning mechanism. Theoretical studies have shown that in active systems with attractive interactions, particles can achieve spontaneous velocity alignment and form clusters through the synergistic effect of self-propulsion and attraction. However, so far, the influence of attractions on collective behavior has not been well addressed experimentally. In this work, an electric-field driven Quincke system, where an electrohydrodynamic (EHD) long-range attraction is present, is used to investigate the influence of attraction on the collective behavior. It is found that the long-range attraction can significantly increase the interaction time during collision, thereby enhancing velocity alignment. The aligned particles can form dynamic polar clusters. Moreover, in the presence of a long-range attraction, a uniform polar fluid is unstable: density fluctuation leads to denser polar clusters which share the same direction of collective motion with the polar fluid. Our findings show that the attraction between active individuals can significantly change the microscopic and macroscopic dynamics of active systems and provide insights into understanding chemotactic attraction phenomena in biological systems.
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